By far, out of all the types of leagues I've ever done (10 team, 12 team, Roto, NL/AL only, etc) in all my years, the one that consistently commands the most of my attention is twenty team, head to head league that I'm in. SOOO much better and more competitive than a 10 or 12 teamer, and head to head is much more fun than Roto, where if you get behind early, it's easier to give up hope.
In the 20 team league, there's much more strategy involved. If a starter goes down, you can't just scour the waiver wire and pick up a capable mid level guy like a Shannon Stewart, Jose Guillen or Mark Loretta, or other guys I've seen on the free agent wire in my smaller leagues. You can still find your hidden gems like a Ryan Vogelsong, or a warm body like Bobby Higginson, but in the twenty teamer, it really requires that you know your stuff for the draft, and makes you work to try to pull off some savvy trades, and anticipate things beforehand. Looking at the pitching matchups, batter vs pitcher stats, and other split stats are all keys to success and keep things really interesting and fun.
No offense, but I think anybody with a pulse and some general knowledge of the sport can pull off a good finish in at least one of their ump-teen 10-12 team leagues, but it takes real talent and skill to be crowned king of a twenty teamer. In my small leagues, I set my lineups a week in advance and if somebody goes down, no big deal, you can grab somebody just as good (in most cases) off the waiver wire, or trade with somebody that has a bunch of good guys on their bench. In the big league, I check it every day, and when inclined spend hours at a time researching and sweating over live box scores. And like in real life, it often times takes days or even weeks of negotiating to get a trade done. But that's the fun of it.
I highly reccommend it to all you guys. Don't know why I don't see more of em, but you really should give it a shot.