I've got 2 85 warrior tanks, 2 85 DK tanks, 2 85 Druid tanks, 1 85 Paladin tank.

You get a -LOT- out of leveling as a tank.

You will learn about pull control.

While leveling is the time to experiment with things like positioning, corner pulls, LoS, Interrupts.

Tanking while leveling teaches you -when- to taunt a mob off someone versus just switching to their target versus letting them eat it for their bad choice of attacking a side mob.

With the right groups, you can learn about CC - and when/how to break it optimally.

You can learn how to be the tank, DPS, and healer all at the same time... a skill that can be done in a lot of leveling content and comes up a lot in progression tanking during that last 1% of a boss when folks start getting foolish or fail to use cooldowns and you discover you're the last one up...
- Everytime you see a tank solo a boss and think how amazing that was, look at the 9-24 bodies around her and think how -NOT- amazing that part had to be before things got to this point...
- No matter how epic your guildies are at not leaving you in that scenario, PUGs and LFR will put you there now and then... And if you -can't- solo the last 10% of a boss, who do you think they votekick first? The 6 healers that failed, the 17 DPS that stood in fire, the offtank that was AFK, or the one tank that solo'd 9% of that remaining 10% before going down... yeah... they vote out the one person who held it together. :D
- Been there, done that, got the bitter pill to go with it.

Tanking while leveling gives you two great things that don't sound so great at first:

1. Forgiving content you can make mistakes in - meaning room to experiment with different methods.
2. The worst possible PUG mates, so that you have to learn how to fix 4 other people's mistakes day in and day out.

If you survive that, by the time you hit endgame, while your offtank is just learning what 'shieldblock' is... you're in greens tanking for the raid and pulling it off.