The story mode would be interesting. I really hope they do what they did with the upgrade to Rock Band 2, where you could rip (almost) all of your songs to play in Rock Band 2. That's a feature that has made Rock Band 2 an investement for me. As long as we can keep building our music collection (as they promised when they first started DLC), I'll be happy. I don't want to be constantly changing (and replacing) game discs.
Hopefully they will sell the game separate from the controllers and the controllers will once again be cross-compatible. The instruments will be a big hit with die-hard Beatles fans, but for those of us who just want a bunch of great music to add to our collection, the controllers are an unnecessary cost.