I don't think what classes everyone is, is going to impact it much. I am consistently seeing players say that they need all 3 classes on their fireteam, and that is just silly.
I will list the reasons why.
1: All classes can use all guns, meaning Hunter does NOT need to be the sniper. Also anyone can use high DPS guns.
2: As of yet, all revealed classes have at least one AOE ability meaning Warlock is NOT required for killing crowds. That and ANYONE can use explosive weapons (Like a rocket launcher).
A: Warlock has a Solar Grenage, and the Nova Bomb.
B: Titan has Fists of Havok
C: Hunter has a wide AOE trap, with lots of shiny parts.
3: In a tactical shooter you enter combat at range using cover to block damage. Instead of hiding behind a "tank."
4: It constrains what players will do, potentially making them miss out of the fun of All _____s teams.
Just wanted people to read this and think about this game is gonna work, and not how other MMOs work.