Killer collaboration tool! I've been running a project to publish a couple of training programs which consist of about 90 1-3 page lessons. We've been managing the whole process using Google Docs and it's worked very well. This discussion feature could really help, though. Sometimes the Comments feature that was already part of Google Docs was a bit too, shall we say, static. First it was a bit hard to remember, for some people, how to add Comments. Discussions is right up on the surface and requires no further clicking to find it. This should help adoption of the new feature.
Also, the ability to close a thread of discussions is great. Conversations come and go and when they are done, you need a way to remove them from the document. Glad Google provided that.
Looking forward to giving this a go. Going to test it out with a coworker today as I do some market anaylsis for another project.