My friend Brian, who plays rythem guitar in our band, and myself were talking about it awhile back--
Brian
I think the main reason Metallica changed was the influence Trujillo had.
I give him his respect and massive props for his skills, but I think he threw a few ideas in about the album.
I
But think about it--They needed a new bassist. We've watched Some Kind of Monster a couple of times, and have you seen Rob telling them how to do the album? No. It was James', Kirk's, and Lars' choices; although I'm pissed at how Kirk wanted to go nu-metal and not have solos. He admitted them himself; so if anyone is at fault, its Kirk.
He simply agreed.
St. Anger didnt live up to the bar because Kirk wanted to change the way they sounded.
He said it himself that 'newer metal is what people are listening to', and that he didnt like the idea of solos.
My favorite albums would probably have to be Puppets, Justice (in some songs), Lightening, and Metal Up Your a** (better known as Kill 'Em' All)--Reason being of the feel...They had the "******** you, we'll play heavy and whip out solo after solo" sense, which I loved.
Before anyone asks; no, our band doesnt go nu-metal. We do know how to play newer Metallica, but we stick to playing the older stuff. <3