Me getting owned.

Improved Arcane Explosion is overrated, Ice Block owns too much to pass up getting it ASAP. I know several mages that went full frost at 40, put the next 11 points in arcane, then the rest in frost. I have actually leveled up a mage full frost through the low levels, it's not that bad, really.

Imp'ed Arcane explosion is pretty much mandatory by about the Maraudon levels, if not earlier. ST and Mara both are substantially easier with a good AoE class, and since most parties bring a mage rather than a warlock, it's pretty obvious who's going to be doing the AoE'ing. Ice block isn't all it's cracked up to be outside of raid situations (Baron Geddon, anyone?) The fire tree offers tons more to help your levelling than the frost tree, unless you plan on AoE levelling in which case you want Imp Blizzard (Which was nerfed, btw) AS WELL AS Imp AE. Ice barrier is also utter shit in PvP, seeing as it's purge/dispellable. Honestly, if it weren't for Shatter, I wouldn't put anything in frost past Ice block. Hell, if MC/BWL didn't reduce the efficiency of a strong fire spec (Though curses and spell penetration help) I'd never have specced frost, it's just not worth the loss of all the damage potential.