Got my new Game Informer, read the ME3 article. Game starts with Shepard on Earth, effectively on trial in regard to the events of Arrival, and in the middle of the trial, the Reapers invade. Prologue involves Shepard's escape to the Normandy, and off to start finding those allies, fighting Reaper troops (husks of all the different species) the whole way.
Confirmed squadmates: Liara, Ashley/Kaiden, Garrus (if he's alive) and... James Sanders... That must be the guy from the trailer, I don't remember.
Speaking of which, there's a screenshot of Ashley with her hair down, and she actually looks hot. Don't have a scanner, so wait till someone with one turns up.
Confirmed to appear, but not guaranteed as squaddies: Wrex, Mordin, Legion, and Anderson. Obviously, if they live.
The game will by default feature a "previously on Mass Effect" comic to make decisions with if starting from scratch.
The Illusive Man also plays a prominent role, and Cerberus is out to kill Shepard. Reason why is unknown at this point.
The RPG elements are being beefed up, not going back to ME1, but giving more freedom with the character skills. Skill trees will be larger, and powers will evolve several times instead of just once, offering even more freedom. Weapons are ME2 style, with a set list, but mods make a return, not in the ME1 style, but allowing you to swap out different parts such as barrels, scopes, and so on, which effect both the weapon's combat performance and its appearance.
All classes can wield all weapons unhindered now, but will have limited slots to carry them. Only the Soldier can carry all weapons at once. Probably means Adept, Engineer, and Sentinal will be limited to two, and Vanguard and Infiltrator will have three.
Also mentioned: The game will have numerous endings, some good, some bad, and who you have in your squad and which allies you recruit can have a drastic effect on which are available.
That's basically the gist of it, except for the sidebars filled with stuff everyone already knows.