I'm jumping in late, but I love UI issues; so here goes...
Here are a few suggestions.
1. Clean out the right-click menus. They're designed to allow the user to do everything. There are about a dozen ways to play a selected song. One or two will suffice. People will say a program is too complex when they see many things on the screen or in a menu that they don't understand. So giving users too many choices can be a bad thing.
2. Make MC an audio player by default, and make an obvious switch to photo album mode or video mode. Show only the functions that are relevant to each respective mode. (You can do this with Tabs and Colors. There are other ways.)
3. Focus on the most used functions. Based solely on my judgement, they are:
Get music: import, cd rip, download
Play music: direct select, playlists, filters
Export music: burn cds, mp3 player
Edit music: edit tags, attach images
I think MC would be better if it were organized around these major functions. And use Big Buttons!
4. Always keep the navigation buttons on the screen. This is extremely important for ease of use.
5. Make it more obvious where you are. Use bigger or bold fonts for location labels (e.g., "Media Library: Audio") and column headers. Use big navigation buttons on the left.
6. Change "Go/Start" to "Help" or "Getting Started". The Help links on the start page are few and easy to miss. Integrate all the help screens into MC. I think someone here provided a good example in HTML. You already have pretty thorough documentation, but many users have been trained to ignore Windows Help.
7. Hide all text-based modifiers for smart-lists and searches. We live in a Windows world; you should be able to do this with control boxes.
8. Don't gray out the toolbar. In Windows, gray means the button is not usable.
9. (You're probably doing this, but just in case...) Do market research. Give MC to a broad range of people (including users of competing products and users with little PC knowledge), maybe a few dozen, and ask them what they think is good, bad, or ugly. You want to cater to the masses.