I've only played for a little over an hour, but here are my thoughts:
1. Vocals seem easier to be mediocre at, but significantly harder to achieve perfection (which is now called "perfect"..."awesome" now ruins your multiplier). Guitar seems no different, except it's harder to tell which notes are "hammer ons."
2. I've only "failed" once (on "Grace" vocals- I just don't know the song), but apparently you don't fail. You just lose all the studs you've obtained. Then they quickly put you back in the game and let you try to win them back. The thing is, I voxtar, and the guitarist is the "leader" of the band. It seems like they give the lion's share of the studs to the leader. At the end of every song, the guitarist has gotten hundreds of thousands of studs, while the singer usually gets around 850. I don't think the disparity in play would explain that, and I play both on expert.
3. I think I like the "den." I like the office- quick way to see your "stats," how you've done on each song. Definite improvement over RB2.
4. The songs are ranked much more difficultly than on RB2. At first, I thought I was missing TONS of dlc, but it turned out it was just listed much further down than I was used to seeing it.
5. That said, at this point A LOT of dlc has not transferred. Of my 200 songs or so, I'd say about half made it. I have no idea what standards they're using to indicate "family friendliness," especially since the game features "So What?" which proudly exclaims that "we're all gonna get in a fight." To be fair, I haven't played this song yet- maybe they edit all the references to brawling. But they already edit the obscenities in the RB dlc. Seems silly to not allow us to play something we chose to download and pay for. Why not have an on/off content filter? Lame.
6. Love the set list so far. "Summer of '69" is a special treat. I can see playing it roughly 300 times before getting sick of it.
7. Like the sequence of the "story mode." It's a nice mixture of getting to choose your own path and dealing with mandatory challenges the game throws at you. RB2 was getting stale for me, even with weekly dlc. It's nice to have goals and progress again.
8. The length of the game will likely be a problem. I can see people who have more time than I beating this in one night and pretty much exhausting all game modes (there's really only one).
9. Bottom line? Songs are maybe good enough that it's worth the $60 total to add to RB2. If all dlc were available, I would probably just save the $10 and play LRB exclusively for a while. As for parties, I can see small groups of people having fun by just starting a band from scratch and playing through, because it's fast and easy. But eventually, you'll want to play all the songs you paid for, and it'll be frustrating not to be able to.