Online is what gamers have demanded because there really wasn't anything else missing from Guitar Hero. It's good to finally have it, but it's always more fun to party in person than via satellite. Battle mode, the new multiplayer option, trades out star power riffs for weapons that hinder your opponent: you must whammy every note, the notes flicker in and out, or one button is "stuck." That sort of thing.
Co-op has a big advancement, and takes a tiny step back. You can now do a co-op career mode, which is surprisingly different from single-player - with an alternate path of venues and a shuffled playlist. Oddly removed is the stat-tracking for each guitarist; it's now replaced with blanket stats.
Visually, this looks truly next-gen on PS3 and 360, but jaggy and foggy on Wii. The characters are well animated and actually synch up with the songs, but this looks like a typical Wii game - which is to say, a little too much like a GameCube game. The HUD has been streamlined, and makes it easier to denote both when you have star power and how big of a combo streak you're rocking.