Flash makes decent points, but there really can't be any serious dispute that they wanted a more radio-friendly mainstream sound on the Black Album than they had before. Maybe calling that "selling out" is a bit harsh, but that's undeniably what they were trying to do. As is evident from the Classic Albums TV special on the making of the Black Album, they were even skeptical of what Bob Rock was having them do, but they had decided they wanted an album that had the sound of Motley Crue's Dr. Feelgood, a big commercial hit, which Rock had just previously produced, so they let Rock have his way. Much of what they did on that album was NOT what they wanted to do "for themselves," as Flash argues,it was what Rock wanted them to do. So in that sense I think Flash has that part of his argument wrong. But I agree with Flash in this sense: To Rock's credit, he did a good job, and they were no doubt pleased by how much better the sound on the album was than it had been on Justice. I don't begrudge them wanting them to make a better living as musicians, but they definitely did change their sound with a conscious desire to have broader mainstream appeal. Plus, they did true power ballads for the first time. Consequently, they have spent their entire careers since then having to defend their choice to be more mainstream and let Bob Rock change who they were as a band. I also think success turned them a little douchey.
Having said all that, I actually like the Black Album. "Enter Sandman" is amazing, and the album does indeed SOUND worlds better than Justice (they added back bass, for one thing). But, as others have noted, Metallica's true genius was on display in the first four albums. I think they were and are talented enough to have continued innovating and producing great music that was NOT repetitive yet stayed true to the spirit of the Cliff Burton albums, while at the same time getting better-sounding production (you know, like Maiden did). Instead the Black Album was a point of departure to make Load, Re-Load, and St. Anger, which affirmatively suck. Death Magnetic was a deliberate attempt to get back to that spirit, Rick Rubin has said as much. And it would have been an awesome comeback were it not for the godawful mastering on the album, which Lars evidently approved. (See my prior posts/thread on the importance of good mastering.). I think the mastering was bad because Lars wanted the music to be LOUD and appeal to the iPod-toting masses -- perhaps yet another example of them "selling out" to the disadvantage of the music and their credibility with their core fan base.
I'm pretty sure Bob Rock could be credited indirectly with making him work more with his vocals than previously. If I find a good source I will put it here 
