I'm wondering if it might be functional to set up different defualt save directories based on the file type that is being downloaded. For example one directory for videos, a separate one for music and a third for other type files. I imagine it might be inconvenient to share multiple file directories but if it can be done this might be worth pursuing.

You can already do this. Go into Limewire preferences and add shared folders.

I have folders just as you describe on my desktop -- one for videos, and one for music. Inside the main folders are subfolders for more organization fun. (Subfolders are automatically shared.)

I used to share my entire iTunes folder as well, before the RIAA went on the rampage. Now I share only "safe" music files.

as berli said, you can already make multiple save directories, but limewire wont automatically send video files to one folder, audio to another, etc. That you have to do manually. i dont think that it would be very hard to program limewire to move video files and audio files into seperate folders, but there are so many different types of files that it might not recognize something as a video file (such as a .zip ifle) and would get placed incorrectly. The best way to do it right now is as berli said:
I. My shared folder
a. audio (subfolder)
1. Jazz (subfolder)
2. Alternative (subfolder)
b. video (subfolder)
c. images (subfolder)

Thats the easiest way, although i do agree with you on needing a organizing system (like in kazaa).