the gear in this game, the sets rather, all have different funtions - and different classes use them much differently than the next!
Take for instance Prot paladins, we tank in av, yet we have no complete tanking set from battlegrounds. Thats an idea! Blizz should make an AV defense set! (I am also not complaining about the badge set - blizz made it up to us)
However if a Prot Paladin takes the say retribution stuff to an endgame raid to tank, or even a 5 man he would have a hard time. Althought it has been done (with more healing and alot of buffs /gems /enchants on the pally, it's not functional.
The sets do different thinga for different classes/specs. One's flaws is another's strengths.