You'll probably get a lot of BS responses to this cause you could look around and figure it out, but here it goes:
1. Both Frost and Fire are very viable. Arcane can work, but is not as realistic a leveling choice for the 1 - 70 grind.
2. Fire is a "Fast Kill" tree. Your goal is to nuke it so quickly and so hard - that it only gets 1-2 hits on you. (For the most part)
3. Frost is a "Slow the mob, control the mob" tree. It has greater survivability, but less straight-up power.
I have been fire from day 1. Basically - I saw that fire did more damage and stuck with it. Looking back I realize that I never gave Ice a chance because I didn't give the spec a chance to show it's control and slowing (plus mana saving) attributes. You will here people say one or the other is better - they are both good. I plan on respecing frost soon. I really want to give it a try and learn how to play it.
Somewhere in my 40s I threw two points into Arcane. I'm now working on the 2nd of the "Improved Fireball"...