Any one level a toon purely because of 1 ability they have

[Thirtyrock]

Paladin tank for throwing the shield.

[Rampant Rabbit]

I turned to my rogue for Shadowstep. Before Shadowstep it was because of Ambush.

[Dietrik]

I leveled my shaman because of chain heal awesomeness in wrath, doubly awesome with the set bonus which gave it a hot, needed a raid healer so was the first choice.

[ShadowJester]

Shaman - for avatar like element control
Warlock - green fire
Death Knight - ghoul
Monk - Fists of Fury

[Slummish]

I leveled a DK solely for the Cower effect Death & Decay used to have when it was glyphed. Man, that ability was so over-powered in battlegrounds. Now I hate my DK for the most part.

[hammock]

Shaman for thunderstorm.

[Ozziee]

I remember I leveled priest only because I liked the spell effect of Prayer of Healing which was originally displayed on the character itself. The Wrath of the Lich King spell effect. They later, in Cataclysm, moved the spell effect to be displayed on the target you're healing, which I think is ridiculous... and still annoyed by.

Yes, I know, I sound crazy. But I don't play the game anymore so it doesn't bother me as much anymore.

[Philosopino]

Back in the old days, I chose my warrior just because of whirlwind

[Pool of the Dead]

Priest because of Atonement.

Healing through damage has always been a love of mine in RPG's and Atonement was the closest I could get to my War Priest from Warhammer: Online where I could equip a 2H hammer and basically became a Ret Pally that healed anything that was near me when I beat the shit out of an Enemy. (10000x better than Atonement could ever be)

[Darkhallows]

Flight Form. And later Symbiosis. It's just so...cool.

Ghost Wolf back in the day. Now I wonder why every Shaman gets ghost wolf. (Pandaren. Seriously. Spirit tiger. Let's go.)

[Dedweight]

There are lots of classes where a particular ability is especially appealing to be, but I've never leveled a toon for a single ability as the sole reason. They'd have to also be enjoyable to play and have other positive features.

I am thinking of dusting off my warlock because of the ability to get a beholder pet. Beholders are fucking adorable. But again, it better have a lot more goin' for it then just a cute pet or I will lose interest.

Same, never leveled a class for one specific ability but rather because it looked enjoyable and I was getting bored with my main.

[Ielenia]

There is no class I want to raise to max level, except for one: a rogue, for lockpicking. Why? It's so damn rare to find a rogue in Stormwind or the Shrine of Seven Stars that is not AFK'ing, and I tend to find lots and lots of those blasted lockboxes, so I usually end up stocked on those buggers before I find a rogue willing to lockpick them all...

[Austilias]

Hm, i've actually never levelled a class for an ability alone. The biggest character I ever levelled for anything in particular was my Paladin when he was a Draenei male called Muradu; namely because of the glorious/bad-ass Draenei Male Paladin in the TBC trailer.

[Arcilux]

Druid for flight form, and Hunter for tame pet mostly.

I mean, I also leveled one of every class just to have one of every class, so I'm not even certain if those two count, but they are the only two examples I can come up with

[Zaelsino]

Not really, but "when the hell do I get Bladestorm" was pretty much the only class-related thing on my mind when I leveled my warrior.

[Terahertz]

DK - Double Death Grip/Strong diseases
Druid - Wild Charge/strong bleeds/flight form
Hunter - Disengage
Mage - Alter Time, pre MoP it was because of Blink.
Monk - Flying Serpent Kick
Paladin - Guardian/ Templar's Verdict
Priest - Leap of Faith
Rogue - Shadowstep/sap
Shaman - Thunderstorm/Lava Burst
Warlock - Portal
Warrior - Heroic Leap

These are all the reasons why I have levelled/would level my certain classes.

[Celista]

I leveled a druid for instant cast flight form and a mage for portals/conjured food (technically not one ability for the mage but still).

---------- Post added 2013-03-27 at 07:46 PM ----------

Not really, but "when the hell do I get Bladestorm" was pretty much the only class-related thing on my mind when I leveled my warrior.

Same lol. Charge/thunderclap were pretty exciting too, I leveled as prot.

[Awe]

Back in vanilla - warrior for charge

[Galbrei]

I leveled a Druid because of flight form a dk because death grip is neat and now a priest due to leap of faith

You... leveled a priest because leap of faith? Isn't that like leveling a rogue because of detect traps?

[Sainiunit]

boomkin because of /dance
Was my first toon, and it will remain my only toon. <3