I have been going through a lot the articles and forum posts trying to figure out rumour and speculation from facts, and have compiled a shot list of what I have found out.
Numbers in  after each line states the reference at the end.
Feel free to add in things I have missed, but a link to a credible source of reference is a requirement.
- Released on 
- Release Date 
- October 9th North America
- October 12th International
- Collectors Edition 
- PC only
- "several goodies like an art book, fold-out poster of the XCOM headquarters, XCOM insignia patch, and even some digital stuff like the soundtrack, special art, and more." - 2K Greg
- Pre-order bonus 
- Classic X-COM Soldier: The blonde, flattop.
- Soldier Deco packs: Aesthetic upgrades to armour including the new Hyperion and Reaper soldier armor kits.
- Including helmet toggle. - Complete color customization: customize your squad's look.
- Difficulty settings [E, 42]
- Impossible (how long will you last -mode) 
- Ironman mode [E]
- Toggle at campaign start.
- Auto-save only, one savegame.
- PC version will have an "enhanced" UI compared to the console. 
- XBOX/PS3, free movement in tactical. [E]
- PC version, grid overlay in tactical. [E]
- "Glam-Cam" on/off setting. 
- Soldier voice on/off setting. 
- Possible HP-bar and damage hide/view in-game setting. 
- Funding Council
- 16 Countries, game over if 8 or more countries lost. [H]
- Funding and resource bonuses varies [12, 16, 36]
- North America, "Air and Space" - All aircraft and aircraft weapons cost 50% less to purchase, build and maintain.
- Europe, "Expert Knowledge" - Labs and workshops cost 50% less to build and maintain.
- Asia, "Future Combat" - All projects in the foundry and the Officer Training school cost 50% less.
- South America "We Have Ways" - Autopsies and Interrogations are completed instantly.
- Africa, "All In" - Monthly XCOM funding increased by 25%.
- Trade & Missions continually 
- Satellite Cover for UFO detection
- Bonus for complete satellite coverage of continents. 
- Satellites can be lost. 
- One base only 
- Hangar bases for Interceptors on multiple continents
- North America, South America, Asia, Africa, Europe confirmed [C]
- Up to 4 Interceptors per interceptor-base. [C]
- Randomly generated mission names. 
- Mission types
- 2-10 types of missions [L]
- Terror missions
- Hostage Extraction 
- VIP rescue, (player controlled VIP) [27, L]
- UFO Crash site 
- Capture alive alien 
- Enviroments [J]
- Alien environments
- Story or "Set type" missions. 
- Large number of hand-crafted maps. 
- Unit placement procedurally generated.
- Mission Accomplished Screen [D at 15:25]
- Aliens Killed (6, Excellent!)
- X-Com Operatives Lost (1, Good)
- Mission Debrief Screen
- New Research Available (Xeno-Biology, Weapon Fragments, Alien Materials)
- Artefacts Recovered (2x Sectoid Corpse, 2x Muton Corpse, 14x Weapon Fragments)
- NPCs confirmed
- Targets for aliens.
- Multiple attacks at once, player have to choose.
- ""There is not just one group of aliens invading, they are hitting 2 cities at once, or three, or four. You have to make a decision, am I going to Hong Kong or ....."
- "..where ever they don't respond to, they have the risk of loosing that country or increasign the panic-meter....." -Casey O'Toole on PAX. [E at 4:16:00].
- Interceptor [D at 17:28]
- One weapon mount (Cannon may be standard side-arm)
- Firestorm 
- Weapons [M]
- Hit% chance- Avalanche Missile
- Fire rate
- Armour Penetration
- Laser Cannon
- Plasma Cannon
- EMP Cannon (less destructive, more loot) 
- "Consumable" items (defence, imp. hit%) 
- Abilities/Stances 
- Item capture and resource requirement for research. [A, 31]
- Each and every research grants some gameplay bonus, not some only narrative. 
Slow (12 days)- Weapon Fragments
Project Cost: 3 Sectoid Corpse
Normal (6 days)- Alien Materials
Project Cost: 5 Weapon fragments
Normal (6 days)- Alien Materials 
Project Cost: 5 Weapon fragments
Hints at body armour improvement.- Experimental Warfare 
- Sectoid Autopsy 
- Thin Man Autopsy 
- Static facilities 
- Mission Control
- Situation Room - funding council.
- Science Labs (?)
- Engineering (?)
- Infirmary (?)
- Expandable facilities
- Alien Containment
- Officer Training School - global perk unlocks for squad
- Laboratories - boosts Science Lab
- Workshop - boost Engineering, "Random resource rebate on whatever you built". -GI
- The Foundry - large manufacturing projects
- Power Generator [A]
- Thermal Power Generator - more power, require steam vent [B]
- Satellite Nexus [A]
- Satellite Uplink - "monitor the airspace over different countries around the world." [A]
- 24 facility expansion slots 
- Bonus depending on adjacent facilities [D]
- Base resource: Power [A]
- Fog of War [3,9]
- Panic [9,12]
- Cover, looks like three types [screenshots]
- Ability: "Hunker down", double defence, penalty to vision and cost one turn. 
- Line of Sight [3,9]
- Day and Night 
- "No gameplay effect for lighting..."
- "... better visibility at night, but more shadows at day." - Jake Solomon 
- TUs replaced with Move-Action mechanics instead. [9,12,13]
- Displayed by two small "play" arrows in UI.
- Two movements possible, UI displays range of Move-Action on ground. [D]
- Inventory grid page removed, replaced with an Ability Bar. [3, 14]
- Still some type of inventory "slots" pre-mission.
- Critical Wounds, possible to revive but suffers permanent penalty to Will stat (panic). [14,15]
- Destructible environments [10,12]
- Walls and Cover destruct able, Roofs or Ground are not.
- Overwatch 
- Weapons explode into weapon fragments [Video A at 4:06]
- Auto-deployment, no Skyranger unload. 
- Doors can be opened in several ways 
- Weapons [D,E]
- Plasma Pistol- Alien weapons disintegrate to fragments upon death. 
- Plasma Rifle
- Heavy Plasma Rifle
- Alien Grenade
- Alive captured aliens keeps weapons intact (required for high-tier tech capture). 
- Ability: Mind Merge, extra health (but can kill both aliens). 
- Ability: Blood Call, group combat bonuses. 
- Two forms [7, screenshots]
- Throws Grenades [J]
- Thin Man (new alien type)
- High mobility, poison 
- Berzerker (new alien type) 
- Muton like, Heavy armoured, Close combat
- Can destroy environments.
- If damaged, will move torwards attacker regardless of turn. [E]
- Floater [29, F at 13:29]
- Ability: Boost: fly straight up off screen and land anywhere on the map. [M]
- Improved grenade ability (Heavy?). [I]
- Heavy Floater [K]
- Chrysalids [G]
- High mobility, can jump multiple levels. [I]
- Units killed by the Chrysalid turn into new Chrysalids unless killed in a couple of turns. [K]
- Alien "Hero" units, (psionic type hinted at) 
- "The Drone" [K]
- Heals robotic units
- Sectopod [K]
- Large bipedal mech unit.
- Squad size start at four and can later upgrade to six. [9,12,20]
- Permanent death for soldiers 
- Class hidden until first promotion. 
- Choice of two abilities on level up (exclusively) 
- Soldier customization [13,14]
- First Name
- Last Name
* earned at Sergant rank. - Country
- Skin Color
- Hair Color
- Facial Hair
- Ability: Squad Sight, fire at enemy seen by any in squad. 
- Ability: Snap Shot, quick shot after movement. 
- Ability: Double Tap, Allows both actions to be used for Standard Shot, Precision Shot, or Disabling Shot. provided no moves were made. 1 turn cooldown. [D at 16:27]
- Ability: In the Zone, additional attack if a critical hit. 
- Improved Overwatch, unit turn does not 'end' as long as he keeps landing hits on aliens. 
- Ability: Supression, reduce target Aim by 30 and take free shot if the target moves. [D]
- Ability: Focused Suppression, reduce target Aim by 65 and take free shot if the target moves. [11, D]
- Ability: Tracer Beam Rounds, if hit alien takes extra damage from entire squad. 
- Ability: Run & Gun, on the turn you activate Run & Gun, you can fire after Dashing. [GI screenshot]
- Ability: Combat Stim, increase stats and reduce damage taken. [M]
- Ability: Smoke Grenade 
- Ability: Rifle Suppression, reduce target Aim by 15 and take free shot if the target move. [14, D]
- Ability: Some type of Improved Heal 
- Ability: Heal Wound, use the Medkit to heal up to 8 HP. [M]
- Late game class [I]
- Ability: Mind Control, Size control of this enemy for 5 turns. [K]
- Ability: Panic [L]
- Ability: Rift, Create a devestating psionic storm. (Free Aim, Limited range) [K]
- Mobile Weapons Platform 
- Takes place of 1 soldier.
- Multiple levels of chassi [J]
- Cover type of chassi [J]
- Multiple levels of weapons [J]
- Laser Cannon [J]
- Sniper Rifle (takes two actions to fire)  - Unlimited
- Assault Rifle - Unlimited
- Shotgun (Assault class only) - Unlimited
- LGM (Heavy Class only) - Unlimited
- Laser Rifle
- Heavy Laser
- Laser Sniper Rifle
- Scatter Laser (shotgun type?) [J]
- Light Plasma Rifle [J]
- Plasma Rifle [G]
- Heavy Plasma Rifle [J]
- Plasma Sniper Rifle [G]
- Alloy Cannon (plasma? shotgun type) [J]
- "Arc Thrower", close range stun weapon. 
- Frag Grenade, Grenades explode on impact.
- Smoke Grenade, (free aim) units in smoke are harder to hit. [M]
Upgrade: stats enhancing drugs for soldiers inside. - Battle Scanner, throw able sight improvement mechanic. 
- All soldiers can wear all types of items and armour. 
- Body Armour - unlimited (start armour presumably) [D at 16:27]
- Skeleton Suit, grapple hook to get to higher ground. 
- Carapace Armour (Requires research) [D at 16:27]
- Titan Armor (Requires research) [D at 16:27]
- Archangel Armour, flying [G]
- Ghost Armour, invisibility, limited charges (3) [G, K]
- "Psionic Enchancing Armour" [H] - Pete Murray
- Temporary patch up a soldier's wound in combat, 2 charges. [B]
D. http://www.twitch.tv/2k/b/314356244 2KGames Pax Stream
E. http://www.youtube.com/watch?v=r4a5biRyFLA Curse cover of PAX Panel
F. http://www.gametrailers.com/episode/...rs-tv/152?ch=3 [at 13:27] GameTrailers TV ep. 516
G. http://www.youtube.com/watch?v=jmrob4oHhZk VG24/7
H. http://www.youtube.com/watch?v=5NAA5tiePKw PCGamer, Pete Murray.
J. http://e3.gamespot.com/live-show/live-show-day-2/ E3 2012 Live Show (at 22min)
K. http://www.youtube.com/watch?v=evnMp_EYnAo IGN E3 Live
L. http://www.youtube.com/watch?v=q5mCcTKBbok E3 Rev2Games
12. http://www.neogaf.com/forum/showthread.php?t=458283 [Second hand accounts, from the GameInformer 226 Article]
13. [GI 226 article].
29. http://www.gametrailers.com/side-mis...-enemy-unkown/ GamteTrailerTV Floater Preview.
30. http://www.rockpapershotgun.com/2012...ties-floaters/ Second RPS Interview
32. http://www.youtube.com/watch?v=4FDel...961&playnext=2 IG Extended
35. http://www.strategycore.co.uk/articl...own-interview/ StrategyCore
40. http://forums.2kgames.com/showthread...e-Soldier-Pack Elite Soldier Pack annoucement.
41. http://www.joystiq.com/2012/07/30/ex...enemy-unknown/ Joystiq.com on Interception
42. http://forums.2kgames.com/showthread...81#post1752281 (thnx to cowfish13)