UMS basically means when plugged in it's seen as a hard drive. I had a MP3 player that didn't, you had to use the included software to "sync" so if you change OS or it no longer works (say in Vista) your MP3 player is useless..unless keep an old PC just for syncing music. Although I guess if you buy a Apple you should be ok.
Rockbox is a alternative firmware/software for MP3 players, basically it's for several Mp3 players, not just for one make/model. There's Rockbox for several Apple Ipods, plus iriver and a few others. Although stock firmware "works" Rockbox has added lots more.
Different font sizes (more lines on the screen at one time)
Themes
Different audio codec support (has added flac, mpc to my iriver over stock)
Safer boot firmware
Rebuilds database on it's own.
Games, text reader
Crossfade, crossmixer...kitchen sink etc.
Adds codec support to Mp3 players, ie you can play ogg and flac on a ipod!
Open source, basically YOU can fix a bug if you're able. Also if you find bugs/request something report them and will be fixed/might be added by another programmer. There's too many things in Rockbox to post here as I'll be all day.
Read up here..
http://www.rockbox.org/