I tend to think that constant message board chat year-round is a good (albeit not required) glue to hold a league together. Our league board is kind of a mini-FFA for the twelve of us, and I think that builds the sort of "rivalry" element that transcends numbers on a page.
I think we reached a point, too, around our tenth year, where we turned a corner and came to appreciate the history that was built up...which was good, because the explosion of internet FF makes for a fantasy football ADD atmosphere.
In a lot of ways it seems to me that you can't support a league if every owner is in 2 or more serious leagues. Sharks and big money players deal with that fine, but I think the key still is to actually feel emotionally invested in your team's wins and losses. That is tough to do with the distractions of, say, 80 players on various teams to track.
Were I to be building a league from scratch, I would seriously consider a no-moonlighting ownership rule. (If my goal was a longstanding league).