One of the features that is particularly noteworthy is that Xen has been updated and now works fully and correctly on the 2.6.26 kernel (see the Debian Wiki page about Xen for details ). This may not sound exciting, but I know that a lot of people put a lot of work into getting this going, and for a long time in Unstable it wasn’t working well. I’ve just upgraded three Xen servers from Etch to Lenny (actually one was Etch kernel with Lenny user-space), and they all worked!
Those three servers were all running the i386 architecture, the next thing to do is to try it out with the AMD64 architecture. One of my plans is to try the latest Debian kernel on the server I use in Germany, but I’ll try on a few other AMD64 machines first.