It's like this baby: Metallica helped define thrash metal in the 80s. With Kill 'Em All, they proved that they could play hard, fast, and with reams of energy. However, the subsequent release of Ride The Lightning proved to be a more thought-out collection of songs. Hetfield's peerless riffing is more refined, and his growling vocals are coming along (he still wasn't too great, though). Hammett's solos are mostly exciting, and he became one of the most copied lead guitarists around (in metal). Even Ulrich, whose drumming was always relatively simple, fits well with the music is gives it a crushing edge. Metallica has never been too focused on bass, so I feel sorry for Cliff Burton he doesn't really get too much attention. He still plays an important role in adding extra punch to the riffs, but one thing that's nice about Metallica's newer work is that the production is more accommodating to the different instruments. (Just a small gripe.)