That's the worst part of WoW. If you let it, the game can easily swallow an enormous chunk of your life for months or years on end. Then, when you finally decide to quit, it's like having amnesia; you can't remember what the hell you used to do with your free time. The whole experience is incredibly sobering and depressing because you realize something so ridiculously trivial had become such an important aspect of your daily routine. Honestly, that's what makes the transition so tough. You're not quitting a game so much as changing your lifestyle.
Anyways, my advice is to combat the boredom with a combination of several activities. Play some games, get out of the house, take up another hobby or three, and so forth. You won't find a single bit that replaces WoW, and you really don't want to - that's kind of the problem with WoW in the first place. It's an adjustment, to be sure, but the one thing you can't do is go back. You'll just fall in again.
By the by, I'm fully aware that a lot of people enjoy a very healthy WoW experience. The game isn't, itself, bad. Still, there's no denying it preys on addictive personalities if those people allow it.
