I think WoW is 5x the fun through level 70.
I would do hellfire/zang/nagrand quests to get to 68
I would also do dungens whenever I could.
some of the very first quests in WoLK at either end are pretty good and close to 5x the fun.
but at some point you will hit an XP wall in WoLK as in some collection quest from hell. ( they are all over the place )

here you might want to boost one or more characters by joining a team setting focus to the boost-e and remove him from the team. as long as he tags first he should get all of the points for a kill.
the elite worms in northern dragonblight are worth quite a bit if your team can kill them, they are 74 I think.