After having read Brice Goglin’s post about what to expect in X for Lenny  the thing that seemed most exciting is the support for Multi-Pointer X . This allows multiple keyboards and mouses with a separate keyboard focus for each. So you can have two people typing two different things on the one desktop.
This should be good for training related to GUI programs and will also have some interesting possibilities for large displays. I wonder if the concept of Pair Programming  could benefit from having two keyboards and mouses. The idea is that one person isn’t actively coding, but following what the active coder is doing may require reviewing other code that is not visible on screen.
Another interesting feature is XACE  the X Access Control Extension. Among other things this is used for Security Enhanced X. Hopefully I’ll be able to find time to work on SE-X in Debian before Lenny is released.