I enjoy leveling up, for the new toys, but not so much the process of getting there.
I wasn't overly impressed with most of the WotLK quests initially, and without a tank on my preferred team, instances weren't a good option either.

I decided to level as many teams to 60 as possible, during the dwindling RAF period.
Towards the end of that, I was playing just to finish the teams and not waste the RAF, not so much because the game was fun.

Typically, because of seasonal work, I take about 3 months off over the summer and don't even use a computer.
This year, I decided to bring my computer with me, and played pretty much all day, one day a week (we work 6 days on, 1 day off).
I really got into multiboxing this summer, going from 2 accounts on 2 PC's to 5 accounts on 1 PC.
But, I didn't take my normal 3.5 month vacation from Warcraft.

The game has become a monotonous grind.
So I'm taking my break from it now.
Like you, if the game isn't fun for whatever reason, either take a break from it or change something to make it fun.
Because that's the whole point of playing a game; to have fun in your spare time.