Things Mass Effect 3 tweeted about its new 4-player co-op

• You can play co-op solo, but it's apparently really hard. 
• Players can drop and reconnect - Shouldn't every game have that?
• There are no time dilation powers.
• There will probably be multiplayer-specific achievements.
• Co-op is server/client based.  
• The classic ME radial menus will be simplified so the game doesn't have to be paused to use it.
• No friendly fire (probably).
• Difficulty uses the same system as SP but scales differently.
• Co-op is spoiler-free.
• Each profile can have multiple characters
• Finally, you'll be able to name your characters - that's awesome.