From the standpoint of a developer that has worked with Wordpress on a regular basis, I think there is one item that Percussion could adopt from them. I really like how Wordpress allows you to build multiple navigation menus.
In my current situation I could really use the ability for this because it would be nice to have a second navigation for our online Policy Manual that I can add to our sidebar. I know I can make a shared asset with a list of the page in it, but the navigation would be nice because then I could take advantage of the breadcrumbs that I’m required to have on all my pages.