since learning by doing is the best way to become a good player, you should choose the spec you want to play in raids later on.
In my case, this is:
prot. once a tank always a tank. and additionally:
- instant pugs (although they may get challenging regarding the catacylsm 5 men)
- always best gear (because you are in dungeons 99% of the time and get all the loot sooner or later, and dungeon rare loot > quest green loot
- no competition with the other 5000 players who are leveling at the same time at the same place.