I've said this in a few other topics, but the commitment problem that [@BrokenPromise] mentioned is one of several reasons why I also tend to only write with those on a similar wavelength to me. I've noted several times that one of the first things I look for when choosing people who I'll allow into my role-plays is their commitment level; in fact, those who've got a reputation of being flaky will usually be denied from my role-plays outright unless they've proven that they've gotten over that habit.

I tend to seek out a dedicated group of players, and I've deliberately made my OOC for the role-play I'm running on this site complex and extensive in order to weed out people who might be lazy (though due to how in-depth I usually am, I'd have very detailed OOCs even if there wasn't that motivation -- those who are looking for something simplistic or shallow should look elsewhere).

With that being said, I've completed [i]several[/i] role-plays in the past, all of which were run by me and were using my main story project's setting. They were completed in the sense that each of them were independent stories or arcs. Given the complexity and size of my main project, it'd be impossible to truly 'finish' a role-play built around that project because there's always the option for new plots (the 'sequel' argument that Broken mentioned), which is what each of the successive role-plays often were. I haven't seen a role-play I didn't run finish to date, but I also usually don't get involved in role-plays unless I'm running them as a general rule.

As for this site itself, I guess time will tell what happens with my current role-play I'm running, but I'm definitely in it for the long run.