One of the way WOW allows for character customization to prevent all characters of the same level being equipped the same and using the same spells is through talents, for which points are gained at the rate of 1 per level after level 10 (so a level 70 character has 61 points to spend). Each class has 3 distinct "trees" in in which to spend points and the mage ones correspond to the schools of magic: Arcane, Fire and Ice.
Arcane talents focus on building up not only Arcane spell damage but intelligence (a ket statistic for mages as it influences the ammount of mana they have to cast spells too) and the ability to resist spells. Fire is mainly focussed on damage dealing, while is is a mix of damage dealing and protective effects. As such, building a mage you really need a mix of everything, and there simply aren't enough points to go round; you're going to miss out on one or other of the trees.
Popular opinion is that Fire talents are the core of player-vs-environment talent builds (or arcane and fire) while frost is at the core of PVP builds. Being mainly a PvE chatacter, I use a "Deep Arcane" Arcane+Fire Build (Arcane for Intelligence and raw spellpower, fire for more forcused raid firepower) though in the past I've been "Deep Fire" (i.e. using high ranked fire talents) and even back before the launch of Burning Crusade a "deep frost" mage (i.e. top-end ice talents). Now, for utility in raids and Battlegrounds I could swap to an Arcane-Ice build (which can be rather nice) but I've avoided doing that so far. The reason? Simple.
The top end tailored garb for robes is quite specific in what it benefits - I spent about a month making the Spellfire set, the benefits of which only apply to fire and Arcane schools. The other tailored sets are even less versatile - the mooncloth set is for healers and the frozen shadoweave is for ice and shadow (shadow being a warlock/priest school not available to mages) and switching would be both expensive (Spellfire is a specific branch of tailoring, so I'd need to unlearn it and then learn the new branch) and hard on materials.
As such, I'm is pretty much locked into Fire and Arcane as my principal schools, at least until I aquires school-neutral equipment from Kharazan and later (or from PvP; I've slowly been buying up PvP gear and will likely pick up some of the Arena kit too). For the time being though, given I'm moderately well equipped, it makes sense to me that others in the guild pick up most of the mage loot before I put in a request for it. There are some nice pieces out that I'll still roll on (from the Karazhan encounters so far there's a cloak and pair of trousers at least I'm interested in, not to mention the necklace I already acquired) and hopefully even more gear will come available as we work through!