Seemed like their biggest concerns are A) getting everything from Rock Band 1 and 2 to translate over to Rock Band 4 (something about 3 is tricky licensing wise it sounds like, probably just the sheer number of songs they put out for it), including the DLC (and for PS3/360 owners to be able to transfer to the same console, PS4/One) B) making sure your old (basic) instruments (plus Pro drums) work on the new consoles and C) refining the three styles of game people play for (score-based, party-based and career-based).
Essentially it sounded like they want to take everything they learned from the past four games, roll all those lessons back to the original Rock Band with a 2,000+ song library (though it's unclear if you'll be able to get any of that stuff if you didn't already own it) while keeping the 'platform' identity of Rock Band 3 where music is always trickling out.
I've also heard rumors that it might be a subscription-based game so you don't have to buy everything individually, that you would buy the game and then pay a monthly fee for access to everything in their store similar to Spotify.