I'm planning to use WP for a Scuba club website, now that I've discovered the Viewlevel plugin. Using user levels, I can subscribe all the paid-up club members as "level 2", while Joe Public can register as a "level 1" user. Each user will be able to post at their own level, or lower, so club members can decide whether they want their posts to be read by everyone, or by club members only - fantastic!
The one feature that I'd like (and which I think has been mentioned before), is a facility for registered users to set up an email subscription to the main blog entries. There's a plugin to allow email subscription to a post's comments, but (afaik) nothing to allow subscription to the blog, so that new entries are automatically emailed to users.
I'm quite new to WP, but not to PHP, so I'm still in the process of working out how to write plugins, plus I've got no experience in interacting with sendmail etc.
If anyone out there can write a plugin which allows users to subscribe to all posts, I can then modify it to use the Viewlevel functionality too...