Thanks for asking Matt!
My few cents (in general order of importance to me):
1. View Schemes - Some way to save the "Showing Columns", column order, sorting, filters, and all of those settings to a specific name and then easily recall them to apply to other lists and tree-views. Also, I would prefer that these "settings" be stored inside the media library they apply to rather than the system registry, so that all clients/computers that access the Media Library in question see the same views (I'm pretty sure something like this is planned, but it causes me more aggrivation ...)
2. Relating to above, ability to set my own "default view" which all newly created playlists, smartlists, and regular tree views show up in automatically UNLESS I specify otherwise (by setting it manually or picking another view).
3. The ability to copy/paste or duplicate Smartlists, including the appropriate "view" information from above. I often make a number of Smartlists that are almost exactly identical, but differ in one small respect. It would be sweet to simply be able to duplicate and then edit the differences, rather than start from scratch.
4. Kind of an anti-suggestion ... Don't mess with my tree-view. I LOVE the easy way this works in MC. I just hide the iTunes-style mess at the top and use the "populate tree" option. Love it!
5. I think going to the "Now Playing" in the tree view is counter-intuitive. I hate to say it, but I think a separate window (ala-RealOne separated) would be better (that way I can quickly and easily switch from what is playing, to the "browser" to tweak the list drag-drop style, and then back again) using the windows task bar. I realize that you can run the player/video window in "windowed mode", but the setting for this is counter-intuitive also - and the window is ugly. I would prefer to see a sort of Mini-Me mode that is always open (which is the "player component") along with the Mega-Me mode (that acts like the browser component).
6. If above is too radical of a code change, simply put a bigger icon at the top somewhere that says "Now Playing" really obviously. I know there is an icon down under the tree-view, but it isn't super obvious to the un-initiated.