Is paladin tanking a viable leveling technique?

[Offhand]

Before you get crusader strike, prot is a way better tree for leveling. You get to pull significantly more mobs and have less downtime. Not to mention prot means instant queues for a random dungeon (and chaining dungeons is arguably faster then questing).

[Hwangchung]

Show me an area where you can get 30+ mobs without them evading

Gathering up a ton of mobs is easy, you just need to do "active" damage on them. That means something you caused, not something like retribution aura. So basically just throw down consecrate and make sure they run over it, they'll stay on you indefinitely. If you're gathering up a massive amount, use rank 1 so they don't die and you have to follow a trail of bodies to loot; almost as soon as you turn around, you'll get all the mana back since so many are attacking you.