Nick S: I agree that was the case for Alias, but you're missing out with Lost, which as of mid-season 3 started to show that it really does clean up after itself. They know exactly how many more episodes the show will have, and they know (to a suitable level of detail) the stories that will fill that. The show is answering mysteries, although it's not always spelling out the answers -- sometimes it's just giving the nerds in the audience all the clues needed to make the appropriate conclusions (or at least drastically narrow the range of hypotheses).

It's also worth noting that Abrams isn't really involved in the day-to-day running of the show, and has had exactly one writing credit since the first season. He helped create it, set the tone and the rules, and he's still an Exec Producer, but the show is now really the child of Damon Lindelof and Carlton Cuse. In other words: Abrams did the part he's good at, and has wisely left the rest in more suitable hands.

There seems to be a recursive structure developing with many of the mysteries on the show. Whereas other shows (e.g., Alias, X-Files) developed the habit of half-answer one set of mysteries, calling it done, and moving on (never to return), Lost increasingly seems to be nesting its mysteries. So, where some might say the show seems to get distracted and ignore older mysteries, it is the case that in the telling of that new storyline/mystery things are revealed that will ultimately be needed to explain the original, bigger ones. In this vein, I think it's fair to say that many of the earliest mysteries will be among the last to be answered... which is probably as it should be.

The posing of a new question tends to attract more attention then the answering of an old one -- especially when the delivery of the answer is understated. Non-nerds see this as an additional, unending layering of new mysteries, but they fail to notice just how much the viewers have learned. It's like when you're stuck in a traffic jam, and the other lanes always seem to move faster.

... comparing the viewing of a TV show with being stuck in a traffic jam probably wasn't the wisest metaphor for my argument.