Okay, I'll ask. Is this impressive or unimpressive? Every reaction I've seen has been some smug "Metallica is playing Blizzcon. *yeah*" instead of people actually saying what they mean.
Sorry, not a metal fan.
Metallica is the biggest Metal band in the world and one of the biggest band in the world...period. So yeah it's a big thing.
What I really mean is that I think it's awesome. I'm not the world's biggest Metallica fan anymore but they know how to kill it on a stage, and it certainly beats getting that band that nobody remembers a year from now :)