According to a GameInformer piece picked up on NeoGaf it appears that Mass Effect 3 won't have multiplayer. It will, however, have several endings.
Also mentioned are:
"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."