Bob Rock worked on every single Metallica album from 1991's Metallica to 2003's St. Anger, having actually played bass on the latter. So it stands to reason that Rock knows a thing or two about Metallica, and Tape Op Magazine tapped into that knowledge in a recent interview.
Rock says Metallica sought him out to produce the massive Metallica album in 1991 after having heard Motley Crue's Dr. Feelgood, and Rock is asked about the drumming styles of both Tommy Lee and Lars Ulrich. Rock compares Ulrich to The Who's Keith Moon and says he's a lot more reactive to the music and conscious about his drumming, and that Lars doesn't "in the terms that most drummers do, I think he thinks in a musical world that is unique."
To me, Tommy, as a drummer, is like an open nerve end, whereas Lars Ulrich is probably closer to Keith Moon than anything. Tommy is a classic back-beat drummer; he is the basis and a rhythm kind of guy. He does have syncopation, but he's a rhythm machine. Whereas what makes Lars Ulrich's drumming so wonderful and so unique is that he's reactive to the music. The Black Album was him consciously trying to be more of a backbeat, keep-the-time kind of guy.
Most of his fills and unique drumming all comes out of the fact that he plays to the riff of the music, much like with The Who. The Who and Metallica are very similar, because Keith Moon played to Pete Townshend's solid rhythm playing, and Lars Ulrich has always played to James Hetfield's solid rhythms. I don't believe Lars thinks in the terms that most drummers do, I think he thinks in a musical world that is unique. He plays to the riff, rather than trying to control the riff.
Like Lars or not, the dude is clearly talented behind the kit and knows a thing or two about writing a song considering who he is today.