Anyone who has ever played any Guitar Hero title for an extended period of time will invariably come across a song that completely owns them.
I'm not talking about a Pat Benatar number, or something from the Killers or Nirvana. They're easy. No, the song that will have you staring in fear at a wall of notes while your rock meter goes into the red faster than ACC will almost certainly be metal, and extremely heavy at that. The reason you never got past hard on Guitar Hero III was because of Slayer, not Santana.
Overjoyed as we were with the news last year that Metallica would not only contribute a bunch of their songs for a band-exclusive Guitar Hero title, we were far more interested to see just how developers Neversoft would deal with the difficulty level. World Tour and Aerosmith were noticeably easier than Guitar Hero III, the former going as far as to require much more accurate tracking for hammer-ons and pull-offs, the latter - well, Aerosmith could be described in a lot of ways, but "brutally hard" would not be one of them.