Being a web developer myself, I get annoyed pretty fast on layout and design that could be better, professional deformity as we call it!
I don't like the use of the frames. It has several disadvantages:
- It looks kinda messy
- There is way too much information visible
- It's harder to make good looking skins
I installed the SubAir application yesterday, which I like a lot! The interface is much cleaner, less information being thrown to the user, etc...
It would be possible to make a similar layout, where current playlist, the whole collection, etc.. aren't always visible at the same time. Tabs work great for this.
To make the player keep player, you'll have to use ajax, which has many advantages.
- Code is cleaner, if you use it to retrieve JSON data
- Might work a little faster
- You can improve usability and add some really cool effects, such as fading text etc...
I know it's very easy to rant about something how bad it is, while not having time to do things myself

Fact is, Subsonic is not bad, it's great! I hope these constructive suggestions might help you make Subsonic even better!