RB2 is as close to perfection as a rhythm game may ever get, and is easily the best game I've ever played.
RB2 fixed ALL of the above problems. I was blown away as I started playing the game, which I had purchased ONLY because I heard it had Breakneck Speed, and nearly every single flaw that so bothered me in RB1 was fixed. Guitar had its share of simple charts, but definitely had a good percentage of challenging charts as well, and it was no longer as though every song below the final tier was a complete snore-fest that I would gold-star sightread. Hammer-ons were made much smaller and more easily distinguishable. Breakneck Speed was introduced, making the more difficult charts actually readable. The vocals engine was completely revamped, making *singing* on Expert as opposed to humming actually viable, and with one exception, fixing talkies so that they usually hit themselves (which makes sense and is much more fun than RB1's talky system). Band World Tour's selection interface was vastly simplified, getting rid of the "band leader" requirement and making the story mode single-player playable.
Anyway, here's my breakdown of good and bad:
Good:
-The aforementioned fixes from RB1.
-Everything meaningful that was good about RB1 is still present. The guitar engine still keeps the very solid GH2 engine, which feels MUCH better than the horrible GH4 engine. Scoring is very sensible and feels quite fair for the most part, and there's the "gold star" score cutoff on each song on Expert for the more hardcore players. All three instruments are just really fun to play, and none have any significant weaknesses.
-Weekly DLC, even three years after the first game. Also the addition of Rock Band Network, which lets even lesser-known artists get their songs available for purchase in RB2. It is now possible to have well over a thousand (maybe 1,500) songs in RB2, meaning that if you're a fan of rock music, there's bound to be stuff that you really want to play in there. The DLC is as nicely varied as the disc songs, and has music from all kinds of genres (even country). The result is that the game experience is practically infinite, since there's always new content for you to play if you want to shell out roughly two bucks a song.
-The game is great at being accessible to anyone who might play it--whether you're a new player who's interested in the concept and wants to get good, an elite veteran who isn't satisfied unless they get the best possible score, or a drunk partier who just wants to mess around, not only will this game be playable, but it will feel like it was made just for you.
-Online play is functional and fun, with no apparent bugs or lag that I ever found.
-All the in-game and menu interfaces are extremely intuitive and well-designed, especially the Quickplay menu, which allows you to sort through hundreds of songs and find the one you want to play quickly and painlessly.
Bad:
-There is nothing substantial that is bad about this game. Not one thing.
Nitpicks:
-Scroll speed scales to the lowest difficulty player, not counting vocals. This means that if one player plays Easy drums and another plays Expert guitar, the guitar track scrolls much too slowly.
-Poorly implemented lag calibration actually changes certain things, allowing for higher optimal scores in some cases. This is only relevant if you're a Scorehero addict looking to get optimal scores, but it's still pretty silly.
-"Strum limit" makes very fast strumming virtually impossible to hit, even if you're strumming at the correct speed.
-It would've been nice to have a mode like Solo Tour in RB1, i.e. the GH1/GH2/GH3 model of solo campaign. The "challenges" mode seems like a poor replacement.
-You only start out with ten unlocked songs out of 84. Granted, this is also what every GH/RB game did up to and through this game, but it's still a little annoying.
-Achievements are noticeably catered towards newer players, making none of them really feel worthwhile. RB1 had better achievements.
In the end, though, the above nitpicks really are just nitpicks, and don't really at all detract from the huge pile of fun that is this game. RB2 is the best game I've ever played. Whether at a party, goofing off with friends, or playing by myself, even two years after the release, RB2 is still something I find myself going back to. It's the only game where I have almost no hope that its sequel might surpass it, simply because it's already so polished that I don't see what they could possibly do to both make it better and not be attacked for "releasing more of the same". So yeah. RB2 rocks, and it rocks hard.