I can attach a comment thread to a location or range of the Thing's display

Summary: This is sort of like the way Wave handled comments, but without the crazy recursion of comments in comments in comments. An attached thread is just like any other thread, but has a location flagged in the text itself.

This is going to be pretty challenging: it will require all sorts of enhancements to understand how a display location corresponds to the source, to keep the location in synch as the Thing gets edited, and so on. It probably will require injecting hidden flags into Display Text, to anchor the threads. But done right, the feature should be spiffy-useful.

Purely a matter of opinion, but the actual display should probably not look like Google Wave -- the way that they actually stuffed the blips inline was not just inaesthetic, it made things downright hard to read. Odds are that the threads should pop up as an overlay when selected, or possibly appear as a bar on the right-hand side a la MS Word.