The people at Bioware have said the demo will be available in january sometime, probably the 26th (ME2's xbox launch day) as sort of a two year anaversery. Additionally they won't just let you try story mode, they're going to throw in coop as well (the only online feature of the game) in the demo as well!
Even if you're not sure on the game yet, it's very good and a surprisingly solid cross genre game. That and the demo is obviously free. Don't dismiss the game before giving it a shot!
Also on more information gathering there's three presets for the game if you're not so story centric and you just want to fight OR you're just story centric ad don't wan't to fight, and an option for being both.
Action mode offers automatic replies in the most standard 'shepard-esque' way and revolve around a normal difficulty curve(to be altered in game), Story mode where your conversations have branches to define your shepard the way you want with a minimal difficulty curve. Finally RPG mode, where the conversations branches are there and the difficulty is amped to normal.
Weapons will be customizable, 5 mods for each type of gun (sniper, shotgun, pistol, SMG, ect.) and have multiple levels of power to alter.
Interestingly the kinect for the xbox offers a more in depth way to play, allowing one to speak their choice from the list as opposed to selecting it.
Relationships used to be one same sex choice for the girl and a lesbian option while the guy had the option of a woman or the bi woman. In ME2 the options were expanded, the only same sex being the female shepard again. In ME3 you're able to be same sex regardless of gender. The option is there!
For gameplay it has always been cover based but every incarnation has gotten better! ME2 plays akin to a grounded UC, and in ME3 they allow blindfire and the ability to pick off limbs/armor pieces to help with killing certain foes. Grenades will be a new option in game as opposed to them being power based, and the melee system is revised to allow killing blows and stunning attacks (i hope to see soldier shepard do a haymaker)
The game is supposed to be the most difficult yet, as AI now have better 'intelligence' as well as squad based tactics over the relatively solo aspect of the previous games.
Story wise, the game has more than 40,000 lines of dialogue (about ME1 and 2's totals combined) as well as 12,500 music/sound files.