The current design of cooldown trading is semi-decent for arena (still boring that sustained damage is not up to par until 80% dampening) but the fact that you are extremely useless without your cooldowns and you have to have them at the ready to score a kill is simply not fun. You feel like a god for the duration of your offensive cooldowns and like a peasant in the downtime.
And the fact some classes crit 20k and above during defensive cooldowns is pretty lame (mainly warlocks, arcane mages and retri palas)