I really believe that Dynasty leagues will eventually be the primary format for most serious fantasy coaches. There just is no comparison when it comes to competition and that feeling of "owning" your own team. After playing redraft leagues (and I still do for fun) and keeper leagues, it just isn't the same level of gameplay in my opinion.
The one problem is that you have to make sure all of your coaches are serious from the beginning. We have never had problems filling vacancies when a coach moves on and, thankfully, it doesn't occur often because coaches that play the dynasty format are usually ... well ... diehards. Expansion drafts are possible as well when you want to increase teams.
Again, true dynasty leagues just don't keep 1, 2, 3 ... etc. players, they keep the whole team year over year. THere is nothing like owning 30 players from an initial draft, then a rookie/FA draft each year. A coach has to really do his/her homework and trades play a large roll. A bad initial draft is costly. But in a dynasty league, if a poorly assembled or performing team can be fun to manage because you completely control the future development in total.
There are only a couple of sites dedicated to Dynasty leagues but any hardcore ffb fan owes it to themselves to give one a shot. Once you go Dynasty, in a good league, you won't go back.
The sites that I use are:
dynastyleaguefootball.com (new but very current/knowledgeable)
footballguys.com (sometimes too much analysis, but good)
nflfreaks.com (free site with good info, they are serious too)