You don't have to press a button to talk to the other team, and the other team can't hear you in game (ie. proximity talk like in Halo 2).
There are grenades, but it's nothing like Halo 2, if that's what you're asking. Now you have to switch to grenades (like you would switch to a different weapon) or you can dual wield it with a pistol or something. From what I've seen, the grenades just aren't that worth it in this game. But I have seen people using them, just not often.
Hope that helps.
