Ghostcrawler has put a significant sticky up on the forums about what he calls the 'hybrid tax' in terms of PvE play -- there's been some back and forth lately on the forums about hybrid classes and what they should and shouldn't be able to do, and GC wants to put any confusion about what Blizzard intends 'hybrids' to be to rest. Very basically, he says that there are three roles in the game (tanking, healing, and DPS), and if a class can respec to perform a different role, it's considered a hybrid class. Otherwise, it's a 'pure' class. This means a few things: pure classes, he says, should have slightly higher DPS ('all things being equal,' and when does that ever happen?), because they don't have the option to switch out. There's no rule as to how much better that is, but as a tradeoff of rerolling being the only way for 'pures' to switch, they get to be a little better. That's the 'hybrid tax,' and mages, hunters, rogues, and warlocks don't have to pay it.
Hybrids, however, do, and that means that paladins, druids, priests, shamans, and to a certain extent, warriors and death knights, will in Blizzard's view never be able to equal 'pure' classes in terms of DPS output, with everything else being equal. You may love your ret pally, and he may be in uber gear, but he should never be able to pour out as much damage as an equally specced and geared hunter, because you can switch to healing, and the hunter can't.