Fire is more damage than frost, but frost has the slowing effect on mobs that it works on, which allows you a longer time before the mob gets to you (more uninterrupted casting time)
Arcane is relatively low damage, but there is a talent that with 1 point lowers your contribution to mob hate, which can effectively increase the amount of damage you can do and not pull aggro (i.e. get yourself in trouble) when you're in an instance.
Also at the bottom of the arcane tree is a talent that makes you an absolute DPS machine for 15 seconds (35% more damage for all spells, but 35% more mana cost). So even though the arcane tree doesn't give much in the way of generic damage increases, it does give you 15 seconds of MEGA damage with a 3 minute cooldown. The fire tree gives signficantly more damage on a regular basis, but can't match the 35% for 15 seconds. So for single target damage you have to choose between good damage increase all the time (to non-fire based mobs) or a mega for 15 seconds with a 3 minute cooldown.
instant cast Improved Arcane Explosion and Uninterruptible Arcane Missiles are VERY nice, and most go at least that far into arcane, which means if you are going to primary in another tree, arcane is almost always the secondary.
The absolute most DPS skill is using Arcane Explosion (talented to be instant) on a big group. I was priest for someone doing this, it was INSANE. He'd run around and make a big train behind him of 6-12 mobs or so. Then I'd heal him once to pull them on me while he cast Flamestrike. Then I'd fade and they'd go back on him, then he'd chain AE while I kept him alive. 10 seconds later there were 6-12 dead mobs and he would drink for a minute and we'd do it again. The biggest limitation is that there are simply not enough mobs in one place to really power level too effectively this way.
The mage is kind of like the rogue in that it's pretty tough to make a BAD mage talent build... you really have to try to screw it up to make a bad mage build.