Here's my WPMU "To Do" list, why don't you add yours:
Bugs 'n stuff:
-Password protections of posts doesn't work in the version I'm using, the post is password protected but no box is displayed to enter a password into to see the box
-It would be nice to be bale to 'add user' without them getting a blog but perhaps it'd also be a potential headache so I'm in two minds over this... if it was built in it'd be great. More info: http://mu.wordpress.org/forums/topic/581#post-3092
-I still can't fix up Drag and drop in IE... the code mentioned here http://mu.wordpress.org/forums/topic/436#post-2530 isn't in my image-uploading.php & the latest version doesn't even have an 'image-uploading.php' so perhaps I'll sit on that one :)
-The Links manager problem needs fixing - but no idea how: http://mu.wordpress.org/forums/topic/596
Goodies & enhancements:
-I'd like to figure out that 'login from the front page' thing from here http://mu.wordpress.org/forums/topic/579 but currently people just end up signing into the main blog
-I'd love to get the most excellent 'spam all users' plugin refined into a high class beast: http://mu.wordpress.org/forums/topic/578
-It'd rock if someone figured out how to integrate Akismet into this!
Come along, share your lists :)