I think one of the things that I had the hardest time with moving from SL8.0 to 8.1 RT was that of the media library. In the SL way of doing things, there's a nice, hard Library object to go find your music in.
As close as I can tell, in 8.1 RT, you scan the file system manually, which probably explains why the Microsoft music app spends so much time scanning the first time you run it.
I finally gave up on trying to figure out how the SL-RT thing goes, and just pitch pre-existing notions entirely, and just start at the RT ground floor. Doing that Quick Start set of videos that MS published recently also helped.