Alright boys, here's the class breakdown in arena from general opinions of the community (which I share as well):
The EZ mode tier (very effective with little effort):
Destroyers
Lyn Blade Dancers
Summoners
The MID tier (can be very effective with some effort, or in the case of KFM with a lot of effort):
Assassins
Kung Fu Masters
The MEH tier (not that effective even with effort):
Force Masters
Blade Masters
What's important to mention is that extra skills points (coming out at 10th Feb and later more) and later level 50 patch will balance this out a bit more. FMs and BMs are both lacking skill points. The effort required to play a class won't change that much, but the effectiveness/potential will balance out, somewhat.