I think Archers could afford to be less... OP, even though it's my favorite class. Either that, or they should be balanced along with the melee and "special" classes like the Chemist and the Hybrid.

Also, I realize Healers are supposed to heal, but they need at least some form of emergency self-defense. Unless you're Tidal with her Fists of Pure Awesomeness, you can't play a Healer when you're the only one left because you don't have any weapons besides your Healing Potions, which aren't supposed to deal damage, and only do so to "Undead" mobs. Healers need at least some kind of weapon that they can use as a last resort. I'd suggest a Wooden or Stone sword with at least Knockback II on it.

Also, a Paladin class would be kind of cool. It could be a variation on the Healer and the Tank. You gain the Healing Potions of the Healer, but have the protective capabilities of the Tank.

In addition, I feel like the Specialist class is a bit... unneeded. I've tried it several times, and the only thing major about it is the fact that it can take more hits from Creepers than any other class. Granted, Creepers are usually the thing that kills the most players in the Arena, but still.