The long running joke about Kirk Hammett is his use of the wah pedal. Some fans feel like it's too much, some don't care, some probably just assume it's part of his sound (I'm in the third category). Though according to Hammett himself in this month's issue of Guitar World, he's never actually written a guitar solo with the wah pedal in mind.
Hammett attributes his use of wah to having fun on the …And Justice for All tour.
"I think a lot of that was just a culmination of having been on tour for …And Justice for All and just having fun on stage, stepping on my wah for a lead break, or in between songs, or whatever.
"But when I really think about the solos on the Black Album, I came up with all of them within about a week. 'Enter Sandman', I had that one complete by the second or third time we played it.
"Because when the songs are strong, the solos come easy. And back then, I'd compose a solo and then, if it felt appropriate in the studio, I'd bring in the wah and see if it brought anything more out. I've never really written a guitar solo with a wah pedal. It’s always been after the fact."