Speaking of plug-ins, there could be some serious innovation in the weblog authoring space if someone married a really nice authoring plug-in mechanism with a good publishing mechanism. For example, Movable Type has a nice, clean well-defined way to create and add new template tags. However, historically there's been no accessible way to do that for the admin interface.
Oh sure you can get in there and change the admin templates or even write your own subclass of MT::CMS but from experience, I can tell you this is not for the faint of heart. Application callbacks are coming along, but still bleeding edge.
This raises a general question about Web apps kicking in my head for a while. Plug-ins and lightweight extensibility are pillars of modern computing applications. Think Emacs, BBedit, Photoshop, Half-Life, Excel, Apache, Firefox, etc. etc. We know how to make desktop and server applications load code dynamically and interface with it. What's the analog for Web applications? How could developers, pro and amateur, write their own Yahoo! Suite, Gmail, Bloglines, TypePad etc. plug-ins? Does this even make sense?
Sounds like something to ask Paul Graham.