I always tank when leveling up If I can, there is no reason to go dps since tanks do more damage anyway and then you have control over a situation and very fast dungeon queues. That changes for LFR though, its a super hostile environment and If I don't already know the tactics and have a strong grasp of the fights from having done normal mode (which is the case, i have not raided since MOP launch) then I wont do it.
In LFR people expect the tank to always know what they are doing and quickly place blame on them, I've seen how bad a tank has been treated for a small mistake resulting from not knowing what to do (and not getting any help when asked), when I'm in LFR (which is rare anyway) I'm looking to just auto-pilot through as a dps.
For normal/heroic mode raiding though, where people are working together and tactics are well explained and understood, I'm well up for tanking and I really enjoy it, my main has typically always played Dps though, as a Warrior. Warrior is the only class I really enjoy anyway.