I agree, I'm a huge Metallica fan and I don't think they're thrash metal at all. A few of their songs are close to it such as All Nightmare Long's guitar riff (which is their best guitar riff IMO) but overall they're just Heavy Metal not Thrash. Take songs like Enter Sandman, Unforgiven 1 and 2, The Day That Never Comes and One.
People who think Metallica are Thrash metal obviously know very little about Heavy Metal and have only heard 1 Metallica song.
Master of Puppets I'm pulling your strings, twisting your mind and smashing your dreams!
Blinded by me you can't see a thing, just call my name AND I'LL HEAR YOUR SCREAMS!