I learned from playing Paladin that many (not all) DPS believe that their positional attacks matter more than avoiding avoidable damage.

I learned from playing WHM/SCH that many tanks underestimate the amount of damage they take trying to pull the entire dungeon; especially when the healer is wearing level appropriate gear, and cannot really keep up with the incoming damage.

From actually playing Bard I learned how incredibly frustrating it is when bards a) don't use Wanderer's Minuet, b) don't use all their DoTs - Windbite exists for a reason and c) forget flaming arrow even exists. You notice these things if you play Bard for a while and go back to tanking, you wonder why you only ever see/hear one or two attacks from Bard when Bard has so many damn options for off GCD attacks.