It´s not very clear how to send a PM. I think It would be more easy if under the profile´s photo (like on Facebook and many other) We put a link for it. Also on the conversations it´s not very usable that the most prominet link take you to your own profile - I click it every first time I go to my "inbox" :(
I think that Vanilla1's whisper feature is awesome. In discussion thread canbe usable like offtopic and like private messages. Necessarily need plugin to realize this classic feature.
Posting comments on a user's profile page seems a lot easier, so adding the ability to make those comments private with a wall-to-wall view similar to facebook (or even making a discussion between the two users similar to Vanilla1's whisper feature) would probably make private messaging obsolete.