It's not often I say “Wow” out loud while reading a white paper, but that's exactly what I did while reading the Xen whitepaper. I particularly drooled over the constructs towards the end of the paper that allow you to - in one easy-to-read line of code - perform an operation on a stream of XML elements, returning a new stream of different elements.
I've been doing a bunch of emacs hacking lately, and although my LISP was never good, it has become slightly less rusty. Xen reminds me a lot of LISP, only with a syntax that's a lot closer to what I'm used to, and that has better support for objects.
I've only just read the paper, so I haven't quite absorbed it yet, but this looks like an extremely interesting piece of software. We probably won't see it for years, if ever, but that doesn't mean I'm not interested.