I got to level 40 and stuff. Thief is amazingly well in this game to me, in most games an AoE for a thief is just an AoE that hits; Yet in this, its AoE has a chance to poison enemies - And poisons do a lot.
It also had 'one for all' type system, where if you got the lucky 50~99% chance to poison one monster, it'd apply to all the ones you've hit.

The tank class is also really good. It lacks damage later on, go figure, but at the start it's really strong; and later on it truly is a tank. I didn't keep my gear up to date and by level 20 things were barely hurting me compared to other classes. The high amount of defense for the class is very noticable.

Thiefs, also have a great critical system. You get passives to increase criticals and what not; Where 1500 Crit = 100% chance to Critical hit - Which, may sound like much, but by around level 40 or so I was around 600~750 critical, since I tried to max that out as I go.
Having the ability to change classes too and then go back to the other sides tree is a great thing, too.
Like, if you're a thief, when you get to 40, or something like that - You can advance into an Archer, which has no AoE's but you gain more critical damage and critical rate; Or go the intended thief way, lose criticals and crit damage, but acquire more AoE's.
Going the archer route offered slower leveling if you solo, but more output in the end.

It's a great game, but... the grind kicked in to me around the 40's that I couldn't be stuffed doing it anymore.