I think the problem I have with Metallica is that, in my opinion, when Cliff Burton died they lost much of their edge. Metallica was ultimately Cliff's band, and the songwriting was original, unique, and powerful. With the Black album, when songwriting was now up to the rest of the band, it becam much more formulaic and predictable. It wasn't terrible, but it was by no means Metallica, it was more like Metallica lite. Cliff gave Metallica all their balls, he was a badass, and when I saw him live, opening for Ozzy, they blew Ozzy away - people actually started leaving when Ozzy came on, but in Ozzy's defense those were his fat and drunk days. At least less so than today.
But that's how it works, it seems. Van halen never went platinum until Sammy Hagar joined, but David Lee Roth's Eat 'em and Smile, a much more vibrant album, was being pretty well ignored. And Aerosmith? Well, they are also a shadow of what they used to be in the Seasons of Wither days. It happens to many bands. I guess you can't fault Metallica.
I was also interested in the comment that Metallica's Turn the Page was better than the original. I felt like they took a song that couldn't have anything else done to it except distortion on the strings and did it exactly the same as the orginal - they put very little "Metallica" into it. On the other end of the spectrum, they cover The Misfit's Die Die My Darling which was so heavy to begin with Metallica could only hope to reach their level. They did a good job with Last Caress, etc., but they couldn't pull it off twice, in my opinion.