I know people are sick of the suggestions to take ideas from ffxi... but ffxi's exp cap system is really what this game needs. a soft cap on exp so without any bonuses a player can't get more than X amount of exp from a mob regardless of the mobs level. Groups of 8 may hit that cap on mobs 15-20 levels higher than they are, solo players might hit that cap on mobs 5-7 levels higher than they are.

Promote's challenging play for groups without seeming to penalize them, and slows down the rate of growth for powerleveling.