Microsoft has released a handy primer for Mass Effect 3 which shows how Kinect users will be able to vocally interact with the game.
Things look pretty simply, but I guess the simpler they are, the easier and more reliable they'll be to use.
Just remember this only applies to actual commands, not actual intentions. So don't just say "kiss lady space Nazi" or "EDI, remind me to feed the damn fish" and expect the game to do your bidding.