I decided to take a closer look at SLiM--the Simple Login Manager--as used by Fluxbuntu (instead of GDM). SLiM is light (about the same as XDM) and it looks really good, even in its default state. What more could you want?
Turns out that while SLiM makes logging into your session easy, it makes powering off or restarting your system from the login screen something of a pain. You need to use "special names" or setup special accounts with special permissions. Either approach is 1) not intuitive from the user's point of view and 2) a security risk. The developers say that implementing a better method is on the TODO list. Once this is done, I think SLiM will be a credible and much more attractive alternative to XDM.