That's the balancing act that bioware needs to be aware of, but as the game is today I don't foresee any problems really. We only have 1 resource pool to "fight against", and not HP, Stamina and Mana which abilites can tie in to.
What could be done with regards to utilizing class abilities rather than the current focus on mdps/rdps, could be something like a 3 second interrupt cyclus, 4 second heal cleans cyclus, stealth mechanics forcing either stealth scan or knockbacks to "get out of stealth", class specific solo encounters - think like a mind trap which presents the trapped player with an enemy specific to their AC - or even the tree they have the most points in (to work with classes that have both heal/tank and dps in an AC).
They have to be REALLY careful about designing encounters that need 1 specific tank type like Shadow w. resilience, or mechanics relying on a Sentinel w. speedbuff or Gunslinger with its shield. It's fine that those make an encounter marginally easier, but the fight shouldn't be MUCH harder without them..
Nod! I see your point, would love to see an encounter that "changes" to accommodate the composite of the group going in rather than the group having to modify to suit the encounter.
The Dev already mention that they're committed to having some kind of puzzle boss., an idea I think maybe cool is imagine a game of checkers with the eight players as the pieces as you move forward your encounter the opposite piece that you have to "defeat" in a one on one encounter to progress where each of the opposition piece is tailored to essentially defeat the incoming group.
But before this can happen some balancing will need tobe done, which is promised for the expansion, so let see.