What Leopard Server features are only fully supported when used with Leopard clients (as opposed to Tiger clients)?
All of the standard services (DHCP, DNS, HTTP) work fine. I can't speak to things like iCal Server or Podcast Producer.
I think most of the differences you'd see in managing clients are marked as "Leopard Only" in Workgroup Manager. Tiger clients bind to a Leopard Open Directory readily enough. I believe you will not have support for nested user and computer groups.
You will need a leopard box to run the server admin tools from. (You can run them on the server in a pinch).
Here is also an AFP 548 thread that someone addresses your question: http://www.afp548.com/forum/viewtopic.php?showtopic=18515
Also, take a look at Tips and Tricks for Mac Management and do a search for "Tiger" -- the author highlights some differences between managing Tiger and Leopard clients using a Leopard server.
The following services don't work at all:
There are a handful of MCX settings that don't cross over as well but they aren't server features and don't apply here.
The services abovie do not work with Tiger client because the client does not support the features. iCal for Tiger is not able to handle CalDAV accounts, has no idea what Time Machine is and the Directory application that will list people and the various information contained in the expanded LDAP schema used in Leopard versus Tiger.
There are also features in the Workgroup Manager preference management (MCX) that are 10.5+ only.
Many features will work with 10.4 clients, but those features work better in 10.5 clients (ie: PHD sync settings, etc).