Their has been a thread going on about the discussion of Classes right on the EverQuest forums so here is the Link: http://forums.station.sony.com/eq/posts/list.m?topic_id=160191
As far as I know to the best of my knowledge, a Hybrid Class is 75% One Class and 25% Another Class, take for example the Paladin witch is 75% Warrior and 25% Cleric witch is the result of a Hybrid Melee.
If you turn that around, say for example a 75% Cleric and a 25% Warrior, you get a Hybrid Caster called a Crusader, personally I like to see all your thoughts about "If Hybrid Melees exist, so should Hybrid Casters" so express your thoughts away.