I've noticed one of the obstacles that seems to stand in the way of getting exploration threads done, and that personally makes me reluctant to participate, is that there's usually several characters that will post promptly. And then there's one or two that post sporadically, only once every two weeks, or never even get around to posting. Since most missions require at least four characters or more, it can slow the process down quite a bit or even cause the entire plot to stall.

While it might not be a popular tactic, I do think it might make for more flexibility if characters get dropped from the mission if they don't post to it in X amount of time (with exceptions made for hiatuses and extenuating circumstances), and replaced with characters that will be a bit more active, and get it done.