WP is by far the most professional blog application I've come across. Keep up the good work!
It would be great to post articles restricted by user level such that posts can be viewed only by registered users with certain user level(s). Password protecting a post is not an efficient approach in the long-run. WP already has a user level system and a feature to restrict posts by user level will be quite easy to integrate I suppose. It would also be neat to restrict access (depending upon the user level) to certain categories. Maybe a "hack" already exists to do what I've just mentioned?!?!
I will play around with WP a bit more. Looks great so far!