I'm still reading this, and thinking, and reading, and...
The idea would be to allow to learn skinning for all people that desire to. Not for just the best of the best, with previous experience, etc. So I'll draw a parallel. Some 5 years ago I believe marko did an awesome job by re-writing the wiki expression page. So those expressions that were explained with absolute, particular values, got to have a complete explanation of their generic syntax. That's what we need here. I don't want to hear in scattered posts what DrawMode does (anyway, leezer3, thanks for that man), I want to know the generic syntax for an entry in main.xml, with all its parameters explained.
And then we need a list of all things that do not exists, but MC will use them if they are user created -> The <ART> section Matt mentions above, the I don't know what other Custom Art folders that MC scans for when changing skins and so on. And whenever there's a change, a new function applied, something new invented, that should be part of the new release notes.
This will help with where we are right now.
We could go crazy later - animations, conditional variables, etc.
And a small note. I'm looking at the latest Noir. While currently minimal, there are platform specific elements. This is problematic, because it's unlikely that somebody will design skins having all platforms in front of him - especially considering how many active skins are in development. And ending up with platform specific skins, or worse, working on some and broken on others... that's major headaches.