Frostmourne is already halfway through its lifespan, so you'll have to decide for yourself if you wanna jump into that or not. I'd recommend just going to another realm and then hopping in Frostmourne in April when it resets.
As for Lordaeron or Icecrown, depends on what you're after. If you're willing to dedicate a lot of time to the game, I'd give Lordaeron a shot. If you're looking for something more casual or if you don't wanna get cockblocked by difficulty hop to Icecrown.
Remember, if x7 experience rate of Icecrown is too much for you, you can always change it:
http://forum.warmane.com/showthread.php?t=433921