First major ME3 details are hitting, it's the cover story in May's gameinformer. This is all spoiler, obviously, but I'll only tag the stuff that's directly story-related:
- No multiplayer (thank god)
- Multiple endings, both good and bad, and companion choices will impact it
- Expanded RPG elements, somewhere between ME1 and ME2 (larger skill trees, powers can evolve multiple times)
- Liara, Ashley/Kaiden, Garrus are all in the game and "playable to some extent"
- Wrex, Mordin, Legion and Anderson are in the game.
- All classes can use all weapons, but only Soldiers can have a full load-out on missions (other classes choose what to take)
- Weapon Mods return, although different than ME1 (supposed can swap out components like barrels and scopes) - it will affect the look of the weapon
Reading on the BioWare forums, it sounds like they're going to try not to spoil the entire game before release the way that ME2 was basically spoiled by trailers and articles...