Well there's 40+ new maps in the expansion at least and they'll be biased to show up more than the old ones apparently so that's something :)

Specific types already freeroam, it's just most of us play at such a slow pace we hit the enemy teleport "feature" I'm guessing the new resource should stop that happening as often as we charge round the map after it...

Not sure they roam the map, but if they do then they need to remove the free turn the computer gets when you first encounter them.

You never get shot at first by aliens that just discovered you, it's always the same animation of a group taking cover wehn you stumble onto them.

Good. Have you ever been shot by activated aliens when you haven't pinpoint their location? It's pretty nasty. Having your entire squad getting ambushed on Ironman mode is not a pretty sight.

The Second Wind Save Scum option and the flanking Angles certainly sound interesting. Not sure if I'd use the angle though, that could be a pain :p

They roam. Occasionally they'll run into your field of vision themselves. However this applies more to the Sectoids, Thin Men and the Chrysalids. Mutons, Outsiders and the Master race tend to stay in once place. Most hated enemy in the game for me were those walking tanks. Huge range, big damage, and one time they managed to bombard snipers halfway across a map where I couldn't even see them.

Yeah, Sectopods are just evil. They even get a free Overwatch move if they didn't fire twice.

They've actually added this now, patrol aliens will get a chance of an attack on discovery apparently

A compilation of information currently known about Enemy Within:

The Meld

Meld is a new resource that is found in containers that need to be captured and it will be present in most maps. Meld canisters have a self-destruct feature that will destroy the resource and damage any player units nearby if not reached quickly enough.
The Meld can be used to create either MEC Troopers (a new class) or to genetically increase any class abilities (including Mechs).
2 new base facilities: Genetics Labs and Cybernetics Labs (which will apparently count as Workshops for adjacency bonuses)

Gene Mods

Alien Autopsies will reveal genetic modifications ("Gene Mods" or "G-mods") for your soldiers which can be applied in the Genetics Labs.
Gene Mods can apply to the following categories: Brain, Eyes, Chest, Skin, Legs, with 2 options available for each (and that can be swapped at a cost)
Brain: Neural Feedback (damages an alien attempting mind control but doesn't reduce its chance) or Neural Damping (soldier immune to panic, becomes unconscious if enemy manages to MC it).
Legs: Muscle Fiber (a soldier can climb walls without a Skeleton Suit) or Adaptive Bone Marrow (limited health regeneration for wounded soldiers).
Chest: a double heart ability to protect your soldiers from death.
Skin: a type of stealth skin that turns your soldiers invisible.

MEC Trooper

With the Cybernetics Lab, MEC Troopers can be created from any soldier, maintaining their rank but losing all abilities they previously had. MEC Troopers will gain an Inherited Ability based on their previous class; Rookies cannot become MEC Troopers. MEC Troopers replace their arms and legs with cybernetic implants and even receive a very monotone voice.
MEC Troopers utilize a Mechanized Exoskeleton Cybersuit aka MEC Suit which can be damaged and require repaired. MEC Suits have three tiers of upgrades, each with options for different Tactical Subsystems; the first tier lets you choose between a Flamethrower or a Kinetic Strike Module, a rocket-assisted punching fist that knocks enemies back (or into exploding vehicles)! Other MEC Weapons: Minigun, Railgun and an energy weapon. Other subsystems include Proximity Mines and a Grenade Launcher.
MEC Trooper Abilities: Shock-Absorbent Armor (starting ability); Collateral Damage (allows to target terrain), a high powered Melee attack, Damage Control (which reduces the damage taken for one turn), One for All (allows them to hunker down and act as cover) & an area healing ability.

New Aliens

Aliens also have two Mech versions, including the Mechtoid (A Sectoid in a Mech suit). The Mechtoid can fire twice in a turn and Sectoids can mind merge with it and give it a damage reducing shield (but it won't die if the merger is killed).
Mechtoids start showing up during the mid-game, about the time regular Sectoids started disappearing in Enemy Unknown; this also means Sectoid Corpses are no longer a finite resource.

Class and Weapons Changes

Two new grenades: Needle Grenade (after researching Chryssalid Autopsy), with a big blast radius but doesn't affect enemies in cover; Stealth Grenade, provides allies with a invisibility paint. Another report mentions also a Poison Grenade.
Mimic Beacon: An item which can be thrown on the map into a square within the player's moving range. The mimic beacon will send out a signal that tricks aliens into thinking it's human, drawing them towards it and anyway from any nearby player units.
Reaper Rounds gives ballistic weapon a bonus to Critical Hit chance.
Assault's Close and Personal is now called Up Close and Personal: gives a free shot when within 4 tiles of an enemy.
Support's Covering Fire now fires before the enemy uses its weapon.
Sniper's Snap Shot aim penalty reduced from -20 to -10.
Sniper's Squad Sight only gives Critical Damage if Headshot is used.
Support's Deep Pockets now gives twice as many charges to Grenades, Arc Thrower, Medikits or any other item with limited uses.
Foundry upgrade that gives all classes the ability to use equip two items.

Gameplay and Interface Changes


New Second Wave options:

Flanking system has been reworked with Aiming Angle: the closer you get to flank an alien, the more its cover bonus declines (works also against your soldiers).
Save Scum: Saving a game will reset the RNG seed for the shots.
Non-cover aliens (Mechtoid and Sectopod) have a 50% chance of firing at your soldiers on sight.
Randomize all Class abilities sets. You may have Heavies with Lightning Reflexes and Squad Sight.

Other gameplay changes

Panic rebalanced to make it less likely for soldiers to fire on their squad mates; panic will include Aim debuff to make those rarer incidents of "friendly fire" less likely to hit.
Critical bonus of stealth attacks reduced from 100 to 30.
There will be a button ("Make item available") in the pre-mission UI that strips all soldiers not currently assigned to the Skyranger team of all equipment.
You can now personalize your soldier's voice using the available audio packs for the current language localizations (French, German, Spanish, Italian, Russian, and Polish).
Memorial Wall will now feature "killed by" information with each name.
There's an option to disable Dr. Vahlen's and Dr. Shen's tutorial voice overs.
Normal difficulty may actually be slightly easier to beat due to the new tech but Classic and Impossible difficulties will be getting harder; in addition to the challenge of new enemies, research times and experience required for soldiers to rank up will be slightly increased.

New Maps

Maps: There will be 47 new maps and some old maps have been "tweaked", some to address exploding cars issues and to add Meld containers.
The old and new maps will be mixed together but the game is set so that you'll see more of the new maps during a gameplay.
There's now at least two new maps that deal with crashed UFOs in a destroyed city and in a farm.

Multiplayer

You can now edit your squad in offline mode.
8 more maps for Multiplayer.

Gotta agree with you, this looks to be shaping up as an awesome expansion of EU, which I really dug. Looks like I am gonna be a buyer on Day 1 as well :D

FINALLY!!! A proper expansion pack!!! Thank you Firaxis!!!

I'm loving everything I'm hearing about this! Definitely a must buy for me.

Nice find...thanks for posting JMich

XCOM: Enemy Within Official "War Machines" Trailer

There are also some stealthy flying squid like creatures in this one, new alien type?