I'm surprised we've had all this discussion of shows losing the plot and no discussion of Babylon 5 has ensued. Clearly, JMS had the whole thing mapped out in advance, but that mapping took quite a beating as actors changed, as the network tried to kill it off after four seasons rather than five, and so forth and so on. Still, despite all the insanity that happened in the real world, the story in JMS's world actually hung together pretty well.

You can say similar things about many of the different HBO and Showtime miniseries over the years. (I'm still bummed they killed Carnivale after only two seasons, although they did sort of vaguely kinda pay off the main tension that show had been building to for those two years.)

As to giving away enough details to reach closure, I suppose JMS is another good example of how this can (or might not) be done. He's got the whole damn world in his head. He could certainly tell us how the plague on Earth is sorted out. But he hasn't. (Damn him!) JMS would probably argue that he choses to keep these things to himself so they retain commercial value later on. Of course, if he gets hit by a bus, we all lose.