Owners that don't participate can be a huge problem. My feeling is that roto or h2h leagues are a bit safer as far as maintaining interest than points leagues. While many points leagues certainly stay exciting until the end, there's a danger that the winner might be determined by the all-star break, whereas roto and h2h leagues tend to maintain the "anything is still possible" feeling a bit longer.
One way to maintain interest is for the commish - or another active player - to put in a bit of extra work and provide details such as player/pitcher of the month awards, update blurbs, and the like. The main factor, however, will always be the people involved. Unfortunately, it isn't always easy to find a group of dedicated fantasy junkies who are also decent folks to boot. In general, people who go to the trouble of posting in places like our classifieds forum are good bets to remain active.
Another thing I've found is that long-running leagues never lose their attraction. There's always some way of improving a team for next year in keeper leagues, even if you're in last place. Even if you don't want to play keeper, merely keeping career stats can help. If I know my last-place finish will lower my average rank, I'll be more willing to put time into that team than if the league will be dissolved in October anyway.
Oh yes - a good commish (or committee, or voting system) is a must. Nothing brings a league down as quickly as controversy and accusations.
Building a great league is tough, but it's well worth the effort. Good luck!