Warriors, aside from tanking/bodyblocking and to some extent dps, are largely party-independent; i.e., they function basically the same with or without a party, and only really benefit from a party existing behind them so they don't have to heal themselves. They have really all the necessary skills (deep wound; bleeding; stance-removal) to make spiking or pressuring just as effective.

Assassins are also capable of soloing without any real penalty. Synergy-wise, they really need another tank in the party to free them up to their potential (free-roaming independent spiker) and can more easily run back to take heat off of casters without worrying about losing the tank; but again, like the warrior, they don't really get benefited from having people behind them aside from the heal responsibility.

Dervishes, on the other hand, synch with more players to a higher degree than the other two simply because of Mysticism. I could go on about prot monks, but the single best example of this would have to be the Orders necro hero, which (with 15 Mysticism) basically translates to one energy regen per second, or three additional pips of energy regen—which is what, six pips essentially? Combine that with Dark Fury, the aforementioned monk prots, and the derv's own enchants, and that's a constant resupply of energy right there, which only the assassin can reproduce (through AP, which is itself prone to failure). The warrior can sort of imitate this through adrenal skills, but it's nowhere near the same magnitude.