Personally I think use of Team chat should be a part of the tutorial REQUIRED to even get your first mission in some kind of 'do this thing with another player' type activity. Just once, and then if you want to go around and play a MMO like a one player game (the point of which is beyond me) then go ahead. I also think that team chat should a) not be uncheckable in the options and b) the bubbles should stay twice or even three times longer than they do.
A team chat tab should be included by default as one of the tabs (but leave team chat active in all tabs). Also, fix the team chat bug when you join a new team - if you were already set to talk to Team, it gets borked and you need to re-select team from the list.