Hob, I don't quite agree with you about leveling specs.

With the newer design philosophy that Blizzard is hoping to use, "leveling specs" may well become things of the past, or at least become less significant.

What is the point of a leveling spec? It is to have the least amount of downtime for the longest amount of "uptime" (i.e. getting experience - not being dead, killing more faster)

Let's look at some of the more popular leveling specs for solo questing:
Druid: You go feral so you don't have to worry about mana. It can be painful until Mangle.
Death Knight: Unholy for more AoE capabilities.
Mage: Frost for Blizzard-grinding/survivability/cheaper spells
Rogue: Combat (I tried to level a rogue as this spec but I wanted to kill myself out of boredom, so I can't say much)
Priest: Shadow for high mana returns
Paladin: Prot for AoE grinding
Shaman: Enhancement for cheaper spells
Warlock: Affliction for multi-mob "DoTting"/better mana returns
Hunter: Beast Mastery for better pet tanking/pet damage
Warrior: Prot for AoE/survivability/Revenge-skillage

Now, many other specs are viable. Smite-priests will be certainly more popular come Cataclysm from the look of the changes to Disc, Subtlety rogues are seen as "more fun" by a growing playerbase (fast Stealth CD, Ambush 1-hit KOs). Many warriors still level as Arms or Fury (MS & Overpower, Whirlwind & fast big hits). BM's status as the hunter leveling spec is debatable, one might prefer Marks' increase to Volley damage. Retribution remains a good leveling spec thanks to Divine Storm.

Players should not be pigeon-holed into one spec just because "it's better", and Blizzard knows this. With Dungeon Finder and BG leveling, you can level your shammy as resto for faster queues in LFG, or make that mage fire and do crazy AoE damage at the EotS flag along with knocking people off the edge.

When I started playing the game as a mage back in 2.1, my friends (who have since stopped playing WoW) told me that "Arcane has offensive and defensive talents, but don't take it unless you're a n00b. Fire is more offense, frost is more defense."
I picked Frost because I thought shooting ice bolts was cooler. My friend had leveled his mage as fire; when I reached higher levels he got mad when I started winning duels.

In short, play because you want to play in the way the spec exemplifies the class. If you would rather shove swords into people, but you want to play a rogue instead of a warrior, go Combat. If you don't want to play a mage, but you love lighting people on fire on your warlock, go Destruction.