One quick note, I own an ipod so some of my suggestions mirror what my ipod does. Here goes...
1. When a track is playing, I think that hitting Exit on the remote should bring you up one level. So that if you are browing by artists, it will go back to a list of all the songs in the album. I guess if you using playlists, it would go back to the list of the tracks in the playlist. This is the same as the ipod.
2. When a track is playing, I think that the Soundbridge should remember which track is playing. This means that if you hit the Menu button and go to the main screen, you should be able to hit select (without using the up or down buttons) to bring you back to the track that is playing. This is the same as the ipod.
3. I own an M2000, and I think that it's screen real estate can be put to better use when browsing. For example, currently if you are browsing artists, only two artists are displayed at a time. Would it be possible add a couple more columns of artists?
4. If you are at the bottom of a list (for example a list of songs for an album) I think that hitting the down button should bring you back to the top of the list. Similarly, if you are at the top, hitting the up button should bring you to the bottom.
5. Make better use of the remote control buttons. I don't think that brightness deserves its own button. That functionality could easily be added to a sub menu somewhere. Maybe instead of brightness, this button could go directly to the current queue.
6. Make use of some type of "cd compilation" flag so that artists that only have one track don't get displayed in the main lists of artists.
1. Fast Forward and rewind functionality. It's not critical, but pretty much everyone expects it.
2. More lossless support, especially flac. I've been using slimserver and on my stereo I notice a difference between the flacs and mp3s that I personally ripped. I already sent an e-mail to Apple complaining about lack of third party support for their drm'ed iTunes files and Apple lossless.
3. I was really impressed with the new radio support in the 2.1 firmware. I'm sure that this is on your list, but windows media needs to get added since it appears to be the most popular format.
4. Some kind of sdk would be awesome too.
Overall, I am very happy with the M2000 and I can't wait for my Linksys NSLU2 to show up so that I can get mt-daapd going.
Long term, it would be awesome to be able to get mt-daapd to serve up lossless files (preferably flac, but I guess I wouldn't complain if Apple got their butts in gear and released their lossless codec) direct to the soundbridge. I can't think of anything better than that combination!