Someone mentioned it earlier and I'd like to reiterate the actual problem:
FREEPS classes are made so they can level up ~equally as fast as each other.
CREEPS are not made anywhere nearly equivalent.
CREEPS more resemble the SOA freeps - where class role was pretty obvious and their short comings were as well.
Currently every freep can do large amounts of DPS AND heal/bubble/use-various-survive-tricks. Creeps are stuck back in the class roles they were issued and since they don't have to level (just rank up).
Personally, if I were writing this game (irregardless of the crying) I would have kept the original class role formula. I.E. minstrels heal, guardians tank, champs AoE DPS, etc. Don't like how quickly you level on your minstrel versus your hunter? Group up, join a kin who likes to level, change your class, etc. However, the change is already there and it most likely won't go back. So that leaves one real option: Blur the class lines further creep side so it more resembles current freeps.