And one specific fact that no one else highlighted. Leveling a 2nd(3rd, etc) class is quicker then leveling a first, because you get an exp bonus for having a higher level class. So pushing one class ahead a little ways usually ends up speeding up the process as a whole(for the bonus exp and all the bonus leveling options that Urth hints at).

As for tanking in a dungeon you dont know... dont worry about it. If you feel self conscious, just tell the group youre new up front. The people who dont want to deal with a new player will leave(good riddance), the rest will cut you plenty of slack(and maybe offer tips and pointers). Most of the dungeons are laid out in a fairly straightforward way, so getting lost isnt typically an issue. And every major patch new dungeons are added, so every few months every tank(and healer and DPS) is put back into the situation of running a few dungeons they dont already know. So essentially, we're all used to it happening from time to time. But if you want to try something different first, by all means have fun with it.