Okay has anyone noticed that the Exp in Grizzleheim is really low? I mean, they ask me to do a 30 minute dungeon for 400 exp...um, I'll pass on that. Myrna Dragoncloud, Grandmaster
Certain spells and school pets rely on you completing GH then WT. The experience GH lacks WT makes up for so it's a trade off. It would off set the experience in general in the game if it allowed you to level there, as you progress in the game your level would be much higher then you may want it to be going forward.
I pace myself with GH and visit an area at a time in between regular questing, usually starting around lvl 25-30. I try to complete GH by level 50 or so. This gives me a chance to enter Celestia and quest in Wintertusk to break it up a bit.