A site which just entered the X-Philes, dionidium.com (created by: Wayne Burkett and J.S. Bell) has made the perfect summary of everything you need to have to be a fully qualified (über) X-Phile: What You Should Know about XHTML 1.1.
I tried to do this myself (though I kept missing things), but I think they have made a great post, especially as a future reference for others. If you know someone who wants to be a X-Phile, just give him this link and they will be fine.