Most people do not like poison, because it takes too long to kill monsters with.
Fire mages have the most raw attack power, but only a few monsters are weak against it (a lot of the high level monsters in ossyria are, and ligators)
Ice/lightning mages are desirable for their ability to either freeze monsters or hit 6 enemies at once, depending on which one you get first. Plus, steelies. The awesomest money making item in the game is yours to get if you become an ice mage.
Clerics are really good, also. But ONLY if you have some good friends to party with. I didn't enjoy playing a cleric when I was in beta unless I was in a party with my friends.
There are many finer aspects but I'm too lazy to talk about them. I wouldn't go poison, however.