Not in the high end game myself, but with characters at lev 32, there arent any I have found (over 6 ATs so far, 1 each flavor) I cant solo. I have found which powers I pick up at which time makes a huge difference in the early game.
Just as in every other game I have ever played, low HP = low survival of errors. A tank can screw up and typically still run. A blaster, defender, controller is already dead from being hit with one shot (or a stun then kaboom).
I think squishies should have status resistance, but tanks should be vulnerable to stuns, etc. Move their resistances over to passives so they survive being held, stunned, etc, and let blasters and defenders have ways to avoid or break free from being affected by status effects. I think that balances nicely, myself.