As some talk is going on about permissions, i thought i should post my sugestion here (two parts):
First, make the permissions for users fully customizable. It would be nice to be able to set which user level is required to access each feature. Presently you can edit menu.php, but it influences only the construction of the menu on the top of the admin screens, if the users tries to access the features directly they will work.
If that is implemented, when one uses the ViewLevel plugin with WP, we could have a really powerful reading levels system too.
Second - less important, and i think most would consider this bloating WP, but i will say it anyway... The prest post status looks like a "post permissions" system: a private post is accessible to the "owner", a public post is accessible to "others". This system could be improved to real Unix-like post permissions, with "owner", "group" and "other" attributes. On a first step, we could have this permissions for reading the post only, in the future they could be managed for writing (editing) also.
This would make WP usable as groupware (not my goal by any means, i am saying this just because other ideas, even for plugins, might stem here).