Priests have to worry about mana again.  In lich king, in full ICC gear, doing heroics was a joke.  Now doing regular dungeons requires actually PLAYING the game again.  I don't have a problem with this, but running out of mana so much again compared to literally never losing mana (unless I was healing the Lich King fight) is a shock to the system.
I feel sorry for all of the scrubs who only have Lich King for reference, and never knew that dungeons in WoW were actually hard at one point.
/nerd