What I have found, when picking up Guitar Hero again, is that I most enjoy playing it on Hard difficulty, the next-to-highest. I can play the game on Expert--I've almost beaten it there, in fact--but I don't usually enjoy that much. Expert is insane. I am pretty sure that it is actually easier to play most songs on a real guitar than to play them on Guitar Hero's expert mode.
I loaded Guitar Hero back onto Belle's PS2 this weekend for a housewarming party, in case we needed something to do. I wanted to have all the songs unlocked for people to play. It turns out that we had 24 people in our tiny little apartment, and we didn't need the entertainment after all. But it was fun to pick up the plastic SG again. Note that I didn't unlock new guitars, or new characters. There's not really much point. They don't do anything except look pretty. Normally, those kinds of pointless unlockables bother me. But here, I don't really care.
Guitar Hero's real strength is that its fun doesn't rely on the characters or guitars. They don't do anything. Your reward for playing Guitar Hero is being able to play more Guitar Hero. This will never happen, of course, but I think I'd like to see the sequel with all the songs unlocked at the start of the game. It would make it easier for my friends and I to just jump into the game, and it would remove the silly system of rewarding people for struggling with a plastic guitar on difficulty levels where they're not necessarily having fun. I wouldn't mind having to work my way up through different venues in career mode. But it'd be nice to pick my own set list.