-Rock Band Music Store: The PSN and Live Arcade interfaces don't really work for serving the amount of Rock Band content that's available, and with weekly updates to continue, a Rock Band-specific interface is needed. Improvments include the ability to organize songs any way you want--including by difficulty--album artwork, and, most importantly, the ability to preview songs before you buy 'em.
-Faster Song Loading: Less waiting around while "loading additional content."
-Pop Cap raised: Maximum number of fans for easier difficulties increased.
-Less Repitition: Less repeated songs in World Tour. So, just maybe, Run to the Hills won't come up every 5 songs or so.
-Improved Vocals: They've worked on the detection of sounds, particulary on the spoken parts of songs.
-Microphone latency improved: More accurate vocals.
-Band Logos visible on Live.
-In-Game text: small improvements made to in-game text.
Sounds awesome, and a Rock Band store was much needed; it was begining to get mighty difficult to navigate the massive amount of content available through the XBL marketplace. The ability to preview songs is also a definite 10 on the awesome factor. I love this game more and more everyday!