WoW supports both these very different approaches to MMOs in equal measure. I've taken each view at various times, myself (although I suspect my periods of hiatus have been born more from the burnout of writing about World of Warcraft as much if not more as I play it at times). But generally speaking, I consider Azeroth a virtual home. I'll do whatever I find interesting at the time, but I'm always there on some level, regardless of what's current and fresh. In fact, I've become quite choosy about what I leap into right off the bat in any given expansion or patch. Some things, I've found, make better leftovers months or even years after their introduction!
If you prefer to play through things from start to "finish" when they're fresh, how long do you expect Mists to take you? How much longer do you anticipate spending leveling up, raiding, and trying out any other features that seem particularly compelling? How long would you like those things to take? What's too long and what's too short, in terms of the lifespan of a major patch or expansion?