I'll get one thing out of the way as we begin this review. Your enjoyment of Metallica's music will be directly proportional to how much you will enjoy this game. It is a love letter to the band, in video game form. Dozens of Metallica's songs, from early in their career to their most recent tunes, are included. The game is packed full of bonus features like motion capture videos, pop up trivia tracks, interviews, and many more.
It is clear that the folks who made the game are fans of Metallica, and there is an extra level of attention to detail, a sort of loving, go the extra mile polish, that was missing from previous Activision Guitar Hero titles. I've not played GH Aerosmith yet, but I have played the AC/DC Rock Band pack, and GH Metallica outshines that effort in almost every way. Whether you prefer disc based releases or DLC, you cannot deny the value of the extras that make GH Metallica much more than just a new set list.
As far as new features go, GH Metallica is not revolutionary by any means. A few little gameplay tweaks have been implemented. There is now a star meter, so you can tell how many stars your performance has earned even as you play. Individual rock meters are a welcome addition as well, so you can tell who is keeping up with the head banging and who isn't. Another small change is a bigger visual indicator when the band is about to fail: the edges of the screen turn red and blink. Each of these changes is minor, but certainly worthwhile.
By far the biggest change is the addition of a new difficulty level, Expert +, on drums. This allows you to use double bass pedals just like Lars himself. (The double bass pedal and splitter cost around $20, incidentally.) I'm not even close to being good enough on drums to test it, but it's a cool addition for those whose calves can take the strain. It's odd that Expert + is available for drums first, and not guitar. Given the fact that guitars have been available in these games from the beginning, a new difficulty for that instrument seems to make more sense, particularly for a game featuring axe virtuosos like Kirk and James.