Disclaimer: The following leveling guide was written by a horsecock futa Draenei, if you think that Futa Draenei have no place in WoW lore please stop reading now.

Seketh until 74
Then Mechanar
When Mechanar goes low, do the TK trio Heroic, and the Hellfire dungeons heroic
Should get you to 78
Then do Icecrown to 80
Hyjal or dungeons until 81
then FOS FOR DAYS

After patch 6.0.2 it's possible to level up at a decent speed by soloing dungeons from the previous expansion. The main Exp comes from the new dungeon completion bonus that you get if you kill ALL the bosses in the dungeon. Along with the good amount of exp you also get a nice chuck of gold, the blue drops from bosses and everything else you decide to loot. All in all the exp and gold you get from the run is comparable to doing quests if not better.

Due to the nature of the whole thing you can only start doing it once you're level 61. How it works is that the bonus damage/damge reduction kicks in once you're out of the level range of the previos expansion's content. Say you're level 71, you're now in WotLK and out of BC, you get a bonus to all BC dungeons making them easily soloable.

61-66/67: Find a fast to clear Vanilla dungeon that's around level 60 range, Sunken Temple comes to mind. [Untested, feedback appreciated]
67-71: You're on your own kid! I'd suggest doing BC dungeons and their quests like you normally would, maybe jump to Northrend and do some questing.
71-77: BC Heroics and late game dungeons such as Steamvaults. Make sure the dungeon you do is as short as possible! Note that lower level dungeons (Ramparts) will give you terrible dungeon compeltion bonuses, that's why you have to stick to higher level ones.
77-81: Same as before, do some quests or dungeons.
81-86: Forge of Souls, easy and fast. Only 2 bosses and the dungeon itself is short.
86-88: Here I did Stonecore, it was faster than Lost City of Tol'vir for that level range. Don't forget to take the portal at the dragon boss once you're running back out!
88-90: Lost City of Tol'Vir, extremely fast, it took me 3:30 to clear the whole place as a Blood DK and while doing those 2 levels I ended up making about 3k gold from vendoring the drops. Don't forget that you can also mount inside the dungeon!

That's pretty much it, if you end up finding anything to improve the guide feel free to copy paste it into a new pastebin and edit the parts. Happy farming!