I'd say "stickify" one of the "What is XML, and how can I use it" threads. It is posted way too many times.
As for interesting threads, some of that obscure DTD stuff about importing entities from remote dtd's is pretty cool, and adding elements to XHTML I'm sure is worth saving somewhere.
Maybe that DTD vs. Schema thread as well.
I could always do it myself (bow before the supermod!
), but I'd like to think you have more authority than me in this forum.