As far as the potential to dish out pain goes, you simply can't beat the Fire tree. This build will allow you to blow things up, and blow them up good. Your primary spells, of course, are Fireball, Fire Blast, and Pyroblast whenever your health dips or Hot Streak procs. Your crits will be frequent and beefy. As always, your survivability with this build isn't anything special, but in most cases you'll be blowing things to bits long before they ever reach you.
When you find yourself in a crowd, your AoE capabilities are the best in the game. Blast Wave will knock your enemies away, giving you a bit of breathing room, and Dragon's Breath is still a very impressive multiple-target option. Living Bomb adds a nice chunk of DPS if you throw it out early in an encounter.
The major downside to this tree, as has always been the case, is that mana still runs out quickly, meaning downtime will be a bit more frequent than in the other two trees. The insane amounts of damage this spec can dish out tend to balance this out, but it remains a concern.
If you enjoy AoE, you could do worse than spending your next two talent points in Firestarter, ensuring you an instant-cast Flamestrike every time you cast either Blast Wave or Dragon's Breath. Following this, I'd spend a few points in the Frost tree, picking up Ice Floes for survivability purposes, a couple points in Frostbite to beef up Frostfire Bolt a bit, and then Elemental Precision for that all-important hit rating. In fact, if you need the hit rating, you might consider spending these points in the Frost tree even earlier, and filling out the Fire tree later on.
This is still the tree of choice for pure mob-grinding. Gathering up a massive pile of enemies and Blizzarding them down still works just fine, and even single target DPS is quite high, what with even more frequent opportunities to freeze and Shatter combo your enemies into oblivion. The spec is very mana efficient, and downtime is minimal, especially with your new and Improved Water Elemental out more often than not.
Your chill effects will occur more often and slow enemies down by more than they ever did before. With the combination of Permafrost and Chilled to the Bone, almost nothing you fight will ever reach you. Not that anything could hurt you if it did--Frost Nova is talented down to a 20 second cooldown and Ice Barrier's downtime is at just 24 seconds. Your elemental is out dishing out damage and pumping you full of mana, your Shatter combos are still deadly, and your ability to control mobs and kite efficiently is still unmatched.
The next two points here should probably be spent maxing out Brain Freeze, seeing as how it's a flat DPS increase, giving you a free, instant Fireball, and proccing with surprising frequency. After those points are spent, though, it's really up to you what you do with the last eight as you level. There are some useful talents left in the Frost tree to pick up, things like Shattered Barrier and Improved Cone of Cold. Even (wince) Deep Freeze might be worthwhile if you plan to PvP, or just want another control/lockdown option while leveling. There isn't much of value to a leveling Frost Mage in the initial tiers of the Fire or Arcane trees, though it's entirely possible you may see something there that suits you.
This build requires level 75, because that's the level you first learn Frostfire Bolt at. The build takes as many of the talents that will benefit that particular spell as it can. The purpose here is to use Frostfire Bolt as your primary DPS spell as you level from 75 to 80, and you won't even require a respec if you wish to keep an elementalist build for endgame raiding. Just progress from this to the level 80 spec I covered last week, and you'll have what may currently be the highest sustained DPS spec for Mages when you step into Naxxramas. I can't speak from experience how useful this spec is for actual leveling, but is this type of build interests you, leveling with it will help you learn it's strengths and limitations as you round the home stretch to level 80.
Whatever spec you choose, and however you prefer to make your journey to the new endgame, enjoy it. Northrend is so massive, so packed with epic scenery, fearsome enemies, fascinating lore, and pure, unadulterated fun, that it's going to take us far longer to see it all than it does for us to level through it. Take your time, breathe it all in, and enjoy the trip. I'll see you in Icecrown Citadel in however long it takes us all to get there, and not a moment sooner.
Every week Arcane Brilliance teleports you inside the wonderful world of Mages and then hurls a Fireball in your face. Check out our recent look at how long your gear will last you in Northrend, or our analysis of the current state of the Mage class as we move into the new expansion. Until next week, keep the Mage-train a-rollin'.