Writing is a funny thing...
I have several WIPs, because I tire of one thing so I take a break and move on to another story. Add to that the various ideas I have, there's a lot I could write.
After NaNoWriMo last year (which didn't go so well), I started writing a story to post on the Holy Worlds fantasy forum. I wanted to see what others thought of my writing, since I never really share much. I was thrilled to death when people seemed to enjoy it.
Having said that... I don't know why, but I've taken a break from that and began the rewrite of NaNo 2009's novel --the one thing I've finished, since it was my first real writing project. A chapter or so in, I realized that with all the changing I was doing, and having gotten a better idea of what I want my world (which still needs a name... >.>) to be like, I needed to do some worldbuilding. I set a goal of writing at least 30K in this rewrite by the end of April, but so far I've only written a few hundred words.
The good thing is that I have been working on said world more lately. It had been a bit neglected, which wasn't very good since it was almost entirely undeveloped. My skill in naming things isn't the greatest, but I have named three of the continents (there is the task of re-doing the map, though) finally. Microsoft OneNote is an amazing tool for organizing all my writing stuff, and it also makes worldbuilding a wee bit easier.
Earlier today I finished summaries of the last two ages of the world. I've found I do enjoy writing history, although the earlier ages are really vague and therefore will be harder to write. The problem earlier was finding a stopping point. Nothing sounded like an "end of the world" that I could be happy with. The war prophesied from the beginning just didn't work out. I ended up with a few people surviving somehow, and begging to not have to wander aimlessly. By the time they faded, there is a small population again. So, there's an ending, but there isn't =P It satisfies me for now, at least. I can stop writing and it doesn't feel unfinished, and there's still room to write more afterwards later (I think there could definitely be some interesting sci-fi stories in that setting).
I've also developed the races a bit more, and I'm still contemplating adding another small planet that is entirely different. It's one of those things that seems like it should (in a writing sense) be there, almost. The thing is, I don't know what I'd do with it.

I'm going to try to stick to my group of WIPs for now and not add any more projects until I get some finished (or, the current draft at least). After worldbuilding some more, I'll get back to this rewrite, and then a different WIP when I tire of or finish it.
But don't we always say that... XD