I've seen a few of us mages still aren't quite sure what specs are good for what and where our powers lay. I know it is nice to stick to what you are used to and changing specs can be a serious drop in performance for a couple of hours (days/weeks/months)
Here is my two cents on what the different specs do for me:
Arcane/Frost:
* Best mana efficiency possible.
* Best threat reduction possible.
* Highest dps (Arcane nuke) if willing to sacrifice range and mana efficiency by going nuts with Arcane Blast (not that viable, but maybe sometimes...) still seems like we are missng a real Arcane dmg spell.
* Good overall dps using Frostbolt nuke with other spells inbetween.
* Good defenses, very good if you take IB over slow, slow can help the group more though.
Frost/Arcane:
* Higher primary nuke damage but with lower crits.
* Best kiting and CC possible.
* Good mana efficiency.
* 10% threat reduction.
* Best defenses possible.
Arcane/Fire:
* Massive crit damage (and ignites).
* Great mana efficiency, rivals frost if scorch is used.
* Average defenses.
* Best threat reduction if specced appropriately.
(note: All Arcane/Whatever specs can do viably high damage with arcane spells at 40% threat reduction to cover themselves when they overnuke their fire/frost)
And the ever renoun 10/48/3
and my personal choice... full Fire with elemental precision:
* Massive burst dps and crowd burst dps.
* Massive sustained dps.
* By far the highest dps for raids.
* Good mana efficiency, bordering on excellent if crit specced. Not as good as a primary-arcane spec though.
* Little defenses.
* DB is sweet as hell, can help alot to turn an angry mob for those few second you need to get away.
Do seem to do a bit more damage on arc/fire, but equally overaggro easily!