The hybrid "tax" need to be bigger. Prolly up in the 15% area. The pure classes are getting lesser and lesser representation. There are around twice as many lvl 80 Paladins as lvl 80 Rogues - only Hunters and Mages(the only really pure dps class currently) can compete with the lesser popular hybrids - upping the tax would make Rogues and Warlocks a more specialized dps class which they should be. Newermind that homogenizing is bad for RPG games.
Up the tax to around 10-15% but ( and this is important) make the Hybrid playstyle emphasize more on offhealing/offtanking in pve.
Ditch meters which solely meassures dps. Hps/dmg absorbed from a hybrid class should count just as much as dps ( if values really matter that much). Thats what hybrid class should "specialize" in - hybrid gameplay.
A team should have midfield players - who both excell in defense and offense, Hybrids is a raids midfielders.