Lars may be an asshole, but it is unfair to say that he made less of a contribution to the writing that Cliff did. Lars has writing credits on 8 tracks of Kill'Em All [0], 8 tracks of Ride the Lightning [1], 8 tracks of Master of Puppets [2], and 9 tracks of ...And Justice for All [3], whereas Cliff only has 1, 6, 3, and 1 tracks on said albums respectively. Furthermore, James wrote nearly all of the lyrics himself, so I really don't see how Cliff can be considered the driving force behind the writing of Metallica's early music.
On the other hand, wasn't Lars the driving force between the terrible mastering of their more recent albums? I remember reading something where he said basically that because he didn't personally hear any clipping and it sounded good in his car that it was good.
From what I understand, Rick Rubin wanted it mastered that way so it sounded "louder" (yet another casualty of the loudness wars) and the band just went with it. Unfortunately, the average consumer (or band member apparently) couldn't care less about audio quality, so it is no surprise that Lars has no qualms about it.
[0] http://en.wikipedia.org/wiki/Kill_%27Em_All#Track_listing
[1] http://en.wikipedia.org/wiki/Ride_the_Lightning#Track_listin...
[2] http://en.wikipedia.org/wiki/Master_of_Puppets#Track_listing
[3] http://en.wikipedia.org/wiki/...And_Justice_for_All_%28album...