I guess this may seem silly - if it's optional, you can just ignore it, right? But sometimes, it's designed to not be easily ignored, and can be quite annoying.

Let's take Arkham Knigth - there's a lot of optional missions and of course all the hundreds and hundreds of RIddler trophies. Sure, you can ignore it, but if you do the game will feel very empty and odd I think. But while some of those side missions are quite good, others are just repetitve and bland, and the Riddler sutff is just an atrocity. And of course, the worst part, unless you 100% it all, including every trophy, you won't get the full ending. I'm not sure if the fact that the ending is not worth it makes it better or worse, but still, it did suck something out of the game for me.

Or God of War - a really good game, but the Valkyrie fights are just terrible. I guess if someone likes punishing boss fights, they're great, but I don't like boss fights of any kind. Even easy ones tend to be much less fun for me than "normal" fights. (And that goes for most games. Normal fights tend to be about skill, and they come an go fast. Boss fights are usually tedious excerises in repetition and memorization.) Sure, I don't have to fight the Valkyries, which I decided on early on, but it turns out they "spill" onto other side content I actually did like. I had a ton of fun with the Muspelheim trials... only to find a Valkyrie at the end. Screw that noise. And why in the hell are the Valkyries much stronger than the actual gods? It's like if Gandalf killed the Barlog and then got his ass handed to him in a one on one fight with some orc lieutenant. That alone annoys me.

I'm sure there are other examples. But the point is - all of that can be just skipped, but then again - it is part of the overall experience. Does stuff like that impact your enjoyment of the game? Or do you just shrug and move on to other content?

Ruin might go too far, but if I think it's badly done, it definitely makes me think less of a game. Example: Last year I played through Dragon Age: Origins for the first time and pretty much despised the optional side quests. It's mostly really terrible mmorpg-style stuff where you just have to kill someone (usually on flimsy grounds), fetch something or visit some locations. Extremely unimaginative and repetitive. And sure, technically those quests can be skipped, but then you lose a lot of gold and experience. I liked other aspects of the game like the lore and even the combat was decent enough, but the nature of the side quests really made me feel that for all its high production values I wouldn't rank it as a top rpg.

It doesn't bother me at all usually unless it's like in Arkhams Knights case where they lock away things like endings behind boring and pointless grinding like riddler trophies.

Optional side quests are perfectly fine for me otherwise.

At least in Origins they weren't just recycling the same few locations for the side quests pretending to be different places over and over, like they did in DA2 :D

While I haven't played the game, I could cite Final Fantasy 10 as an example, specifically the effect that the celestial weapons have on the superboss balance.

Specifically:
* Celestial Weapons *ignore* defense. This makes them incredibly powerful in all cases, to the point where, even against high defense enemies, they are far better than any spells (at sufficiently high Strength). The result is that attack magic is useless against such bosses in comparison to just using normal attacks (or a certain skill that works like a normal attack, except your next turn comes more quickly).
* There's also the fact that you can't customize them, so everyone is using the same set of abilities on their weapons for the superbosses. You can't choose to give a specific character's weapons abilities you like more, because the abilities on the celestial weapons are fixed. (Also, even if you could, Break Damage Limit is basically required, because (at high enough Strength) your attacks deal 10x as much damage with this ability than without it.) (Fortunately, there's still room for armor customization, and it turns out that whether to use Break HP Limit is still a reasonable choice.)
* The superbosses are balanced with the assumption that you'll have celestial weapons, so you basically have to acquire and use then for these bosses.
* And that's even before the topic of what you need to do to get these weapons. From what I understanding, some of these tasks are unfair and require doing things that a (nominally turn-based) RPG should never require (creating an accessibility barrier).

So, in this case, the existence of some optional content results in other optional content that is balanced around doing it and hence ths first bit becomes required for the second, and the second is hurt because of this.

The Final Fantasy series has had other issues as well. For example, in Final Fantasy 7, who thought Knights of the Round was even remotely close to being balanced? (Final Fantasy 6's Ultima has a similar issue, but at least you need to combine it with another spell that takes work to get (Quick) and an accessory (Gem Box/Soul of Thamasa) to break the game; FF7's KotR breaks the game *by itself*.) I hear Final Fantasy 10 International/Remaster has a similar issue with Dark Aeons, which can cause problems when you try to revisit those areas (they're the superbosses I was referring to); without Celestial Weapons and high stats, beating them either isn't realistic or requires a lot of time (or Zanmato, but then you don't get to fight them properly later, and you just spent a lot of money on one boss).

Edit: Why the low rating? What's wrong with my post here?

I have docked points absolutely for side content. It especially depends on "How side" the content is.

In example, Slime Rancher, where the game pulls a fast one and tries to make the ranching look like the main activity, but it's secretly a side.

A side quest that gives you permanent increase to your running speed? I'm going to sideye the designers and wonder why a quality of life feature wasn't included by default.

Something like a skill that while not needed to complete the game, can help optimize the flow of battle? I'll wag a finger and wonder why it isn't in the natural progression.

Some weird sidequest that gives an uncommon item? Eh, not so bad.

Completely pointless wild goose chase that only exists to make fun of sidequests? Very bad.

So as you can see, it all depends on what that side content is.

I can't think of a game where it detracted from the overall experience no. Usually I think I quite like it. For example the optional side missions in the GTA games, and even the minigames like being able to play pool in GTA:SA or go bowling in IV. I also really enjoyed the side content in Atkham City because I loved having another reason to explore the amazing setting. Haven't played Akrham Knight yet however, but I can imagine it becoming grating if the riddler stuff is poorly designed.
Tiberian Sun is an interesting example of an RTS doing it right: before a lot of missions there is an optional side mission, which can make the main mission easier if completed. E.g. destroying Hydro dams that power the perimeter defenses of a the enemy base in the main mission.

But to truly answer your question I need to think of an example of where I didn't like the optional side content, but for some reason I cannot think of an example right now :P

It very much depends. I am currently just about finished with AC Valhalla (not a bad game, fair few bugs, bit bland, combat average, very pretty) and Ubisoft are the prime proponents of the copy paste 3 or 4 items across a wide map, with Valhalla being no exception. I quite enjoy them to start with, however they quickly become a chore, and in most cases immersion breaking. Syndicate for instance has chests on every back street in poverty stricken London, and the Caribbean in Black Sail also has something to collect on every tiny island you come across. If they released mod tools I would remove all that. The thing that annoys me most about those games though is the modern day parts they push into each game! Far cry is the same as is Fenyx. It’s just the way Ubisoft is.

With open world games it is a common thing though, it’s just too much work to fill each area individually. In some way Ubi have got it right by doing season passes and adding specific content which flesh out the areas as they go along. It actually makes the areas feel a bit different.

Can’t say I had any issue whatsoever with optional content in origins, game is just fine. DA2 was pretty bad as you say. Inquisition was better than 2, however that was very much the same cut and paste open world mmo type thing.

So I would say yes and no. Being someone who likes to clear everything it can get annoying and in some cases immersion breaking.

Depends on the game. My pet hate is mindless grind / "farming XP" purely for the sake of it. In many games it's just "obvious filler" for lack of main content. Someone above complained about Dragon Age Origins and yet the main quest vs side quest balance in that was pretty good. It was Dragon Age Inquisition where almost 75% of the game is "sidequest spam". I do prefer side-quests that put a little more effort into being creative though.

In Arkham City (I haven't played Knight yet), I just played (side missions / Riddler / training*) what I liked and skipped or interrupted what I didn't. In the end, I felt that I played more than enough worthwhile content.
Sure, hiding the "best"(?) ending behind a 100% requirement is a bad thing to do - but I'd just watch it on Youtube and move on.

*I skipped the harder training missions (glide through hoops). I find little fun in hand-eye coordination / reflex challenges.

What is 'optional'? ;)

I like side content when it is:

Interesting side quests or exploration sites that add flavour or some background to the game world. On the other hand, mini games, races or repetitive side tasks can ruin the experience for me.

Meant to be optional, even if rewarding. No grinding. I.e. no collectibles or achievements trying to force the player to complete 100% of the side content.

Possibly. Chrono Trigger is one of my top 5 all time favourite video games, and the DS port made me not want to play it again for a long time because of how piss poor the optional after-game bonus content was.

They added two new side dungeons to the game, both of which added irritating, nonsensical retcons to the story. The Dimensional Vortex was simply a forgettable asset flip, but the Lost Sanctum? Good god, it amounts to little more than making you run up and down a mountain across multiple time periods for what felt like an eternity doing little more than fetching things and talking to npcs, all while fighting the same respawning enemies up and down the path every single time, it was such an obnoxiously long, mindnumbing slog that for a while there it burned me out on my favourite RPG.

While in the long run it didn't ruin the original game for me, you couldn't even pay me to play the DS version again.

Also, I would tentatively say that Xenoblade's side-quests could potentially do this for some as there are just so many small, sometimes mindless side tasks available everywhere, and at some point late in the game I got distracted by them and began to start obsessively doing as many as possible, to the point I forgot about progressing the story and accidentally overlevelled myself to the point the final boss fight was a total anticlimax. But I think this is more on me than any real fault of the game.

If it's truly optional it doesn't bother me but if it's kinda mandatory in that the game is going to be brutally difficult if you don't do the extra content then it does bother me. Having to do some side content is okay since you are supposed to engage with the game but generally speaking I like optional content to be optional and not "mandatory wink wink optional".

I could argue that the ret-cons took place in another dimension, and therefore there's still a dimension where the original plot took place and the new content did not.

I could also explain away Chrono Cross with the same "it happened in a different dimension" argument; there's still a dimension where Chrono Trigger and Chrono Cross took place.

(Once you go through a place called the Dimensional Vortex, it's reasonable to argue that you are no longer in the same dimension, hence anything that happens at the end of it happens in a different dimension.)

By the way, I am reluctant to play Chrono Trigger again, but for an entirely different reason; the mandatory button mashing segment that occurs while in prehistoric times (after a boss fight, but before the next opportunity to save, IIRC).

Edit: Forgot to ask this when first making this post, but why was my previous post in this topic downrated?