Wow, I... I almost wonder if Torment isn't being called out just because it's considered to be so "perfect" by so many people?
Anyway, I already posted my disappointment with post-Privateer space-sandboxes, but why not add in a few more distinct disappointments:
The first was an XBOX game whose name I can't remember, but perhaps someone on here will. It was a third-person hack'n'slash RPG-lite, and a highlight of the game was that much of the environment was destructible. I remember the game being hyped that you could "destroy anything", "shape the world to your whims", and that there were multiple endings that were decided purely by your actions. However, it didn't really deliver on any of that. The game was buggy, hit detection was terrible, the controls were atrocious, and there actually ended up being few destructible elements in the environment, and rarely did they offer an actual tactical use.
Next is the Hacker series of games. Each one promises to deliver the feeling of being a hacker, breaking into all manner of protected data, siphoning funds, and controlling the world from your computer. Each game also attempted to deliver this through different sets of mechanics. And each one failed pretty miserable. The games seemed to progress from a game with some promise, seeing as you actually did have to use the keyboard and type commands, to a point-and-click strategy game with a "hacking theme" in the last installment. Ultimately, Uplink did it better. It wasn't perfect, but it was more cerebral, and actually did reward you (AND was a little easier) if you actually knew a little bit about real-world "hacking", which really amounts to just knowing some Linux terminal commands.
Star Trek Online. I REALLY wanted to love this game, but the combat is just too fast. I was really hoping for a game with methodical, tactical, slow-paced "submarine warfare in space", like in Wrath of Khan. I get that slow-paced and MMO don't really go hand in hand, but I really haven't seen a Trek game do this right, at least not in a long time. Yes, I know that battles between ships in more modern Trek media have been portrayed as having a faster pace, like the Borg Cube assault in First Contact, but... that's still not what I think of. If I can't really get into the starship combat, then there's really not much to keep me playing.
Having said that, I should add Starfleet Academy. Don't get me wrong, I still loved the game, but... it's basically Wing Commander III reskinned with Star Trek ships. The ships move more like fighters than big capital vessels. I played the game expecting a simulation of being a Starfleet Captain, and at a time before Trek got all whiz-bang with the fleet battles, when ship confrontations were still at speeds and distances where The Picard Maneuver actually worked. At these speeds? Well... considering the size of the vessels, and the distances they seem to travel, and in the time they travel them... I would estimate 1/2 Impulse to be about twice the speed of light. Who needs Warp Drive?!
This is a bit of a stretch, because it is less about and idea versus execution, and more about a terrible "re-interpretation", but Doom 3:BFG Edition. This game was supposed to be the penultimate version of Doom 3, and it wasn't. Period. About the only advantage was widescreen support out of the box, and allegedly higher resolution textures, with the inclusion of the expansion packs. I did enjoy the expansions, particularly the otherwise unreleased one, but the primary thing that made Doom 3 so enthralling was the shadows, and the need to choose between seeing and shooting. You couldn't hold the flashlight and the gun at the same time. You would find yourself turning a corner, and your flashlight would catch some object and project a distorted shadow on a wall. Next thing you know, you're crapping your pants, and blindly firing your gun in the dark. Meanwhile, an imp has crept up behind you, and proceeds to gnaw your arm off. There's just a LOT missing from the original game in this version. I find myself just wanting to play the original Doom 3 with a widescreen hack and some graphics mods. Truly, the original IS still superior!