Mage: 3 types of magical damage -> 1 school will always suck, most times 2 schools suck
Warlock: 2 types of magical damage -> Shadow for all specs, Fire if your raid hasn't got spriests
Druid: 2 types of magical damage -> Nature for DpS, Arcane for DpM
Priest: 2 types of magical damage -> Shadow for dmg, Holy for healers
I'd definately give all my warlocks fire spells just to get rejuv or lifebloom, even with just about 1k addheal from the spelldmg gear would definately rock. And puuullleeeez let's trade roots and nature's grasp with fear too, if we're just building some funky castles in the air I'd love it!
No "pure" class, like "pure melee" Warriors/Rogues, "pure ranged" like Hunter/Mage/Warlock, will ever be ok with the versatility a hybrid class gets. And if the classes can be just as effective in their "hybrid" part as a pure class is. You always keep about 25-30% of your "hybrid" part. Healing in feral stuff suxx, but you can still heal for about 10-13k more than every other melee could, as balance specced druid you even keep some more of your healability and a healer can also just do his gimp melee or caster dmg.
I know these are just castles in the sky, a druid is a very cool trash tank, deals ok melee or caster dps but the only thing he can really shine at is at the healing part, and this is where all the hybrid classes meet. They are healers that can fill different other roles they just loose more and more of quality in their "other roles" the more the itemlevels rise. Cats are nice DpS, but no match for rogues or furies. Bears are nice tanks, but crushing blows from biggies like Teron are just too much. I dunno about boomkins, never met one in a raid.
From outside the box it just seems to be sooo unfair, but every druid can tell you that it's not possible to do everything at the same time.