I just beat Medium with no problem. Most of the songs were five-starred the first time, and the ones I missed are probably because I'm actually too fast for Medium and because my brain hasn't adjusted to a non-laggy HDTV signal. (Also, I got a flu shot today and feel like crap...but that's another story.)
I went over to Quick Play to try a couple of things on Hard to assess how hard the next level would be -- five-starred "Girlfriend" at 97% on the first try, and even five-starred "Jordan." If people are saying that GH2 is more difficult...it must be either in the uppermost tiers of Hard, or on Expert.
As for the song list, there are some tracks which REALLY bug me, which is something I couldn't say about the first game. RATM sucks, and "Killing In The Name" is an incredibly boring song to listen to...much less play. "Heart-Shaped Box" is a bad cover of another boring song. A lot of the newer metal in the game isn't familiar to me and I didn't care for it much on a first listen. Then again, some of the older metal isn't that good either -- "Madhouse" is a terrible choice for an Anthrax song. "Them Bones" wins the "Take Me Out" award for "great song with a song chart which doesn't quite match what you hear and screws you up."
The highlights of the set list are awesome, though: "Surrender," "Strutter," "Message In A Bottle," "Carry On Wayward Son," "Search and Destroy," "Cherry Pie," "Girlfriend," "Sweet Child O' Mine," "Bad Reputation," "YYZ," "Institutionalized." Of course, "Hangar 18" is an inspired choice for the penultimate song -- I love Marty Friedman's guitar work, and that song is just MASSIVE. I suppose it's inevitable that "Freebird" is the ultimate song in the game, but I have never cared for the song very much. (Yeah, I know, sacrilege.)