Just don't suck.

All classes can look good.

Also on the flip side of the coin, its really, really easy to be a "good" healer if the rest of the group is good - if noone is taking damage, well, you can afk cure the tank.

Similarly, a craptacular tank just going 1-2-3 can be saved by a good healer or if dps can burn through enrages etc even if the tank knows nothing about cooldowns.

All roles can suck pretty bad.