I've only played in hard mode with one other real player. No elitism witnessed as the heroes don't care if I have a decent bar, or just healing breeze and rebirth.
From using heroes though, I can say so far that... paragons, ritualists and mesmers are extremely helpful to have in parties. Cleared Perdition rock with a paragon in the party, was extremely helpful. Cleared Witman's Folly with a ritualist in the party, was extremely helpful. Have cleared several areas with a mesmer, and the little fat man has more than done his job. I have not tried assassin or dervish. Not because I'm particularly adverse to it, just that mostly we got on a roll and didn't want to set up new bars/heroes to get going.
Monking is not so bad as long as in 8-man parties, the 2-3 monks have good builds or even if you're the lone monk, you have a good build. Good tanks are extremely helpful. Minion masters are extremely helpful in parties where you're restricted to 4 people and SS and SV absolutely dominant. Fire nuking is dominant as well. It's really not as easy as just saying "the best and worst classes for hard mode".
In areas where you're extremely restricted by having a party size of 4, build tweaking might be necessary to find the balance between damage, interrupts and healing.
Finally, golden eggs and 4-leaf clovers are your friend.