I look at it like this, and Metallica and Megadeth go hand in hand right down the road.
When they started out, they were young, full of piss and vinegar, and barely could keep themselves fed. James was getting out a lot of anger towards his father and fundamentalist Christianity, which he grew up with. They were all hungry and piss poor, and that hunger showed in their music.
As they got older, more experienced and more popular, they realized that Elektra was basically bending them over a barrel as far as money goes. They might have made all their money on the road, but they were not seeing a good percentage of it from their albums sales. That made them angry, and they focused that anger by putting on some of the best live shows in the mid-to-late 80s.
The Black Album is the lynchpin. I for one have never believed that they sold out on this record. It was a big departure from what they were known from before, almost too big. But the album retained enough of the "old" Metallica that after a time, it was embraced. There are maybe one or two tracks (in my opinion) that keeps this album from being in the rarefied air of Ride the Lighting, Master of Puppets, or even ...And Justice For All.
The writing was on the wall when Load was released and they were doing interviews, and they had gone alternative and cut their hair. By this time, or around the time of Reload that they had finally gotten out from under that contract and signed a better deal with Elektra, But now they are well into their thirties, and I am sorry, but you cannot maintain that level of anger that fueled some of the best albums in the history of rock music. So the way I see it, Reload was the downshift. Load had enough good tracks on it to still decent, though it still falls short of the impact that The Black Album had. The turning point for me when I was reading an interview with James and he was asked how the fans were digging the new music and basically what he said was that they were *making* the fans dig it. At that point, I realized that Metallica had crossed a line.
Then all of that crap happened around the recording of St. Anger, some of which had been a long time coming. Jason leaves the band and only in hindsight do James and Lars realize how shitty they treated him. The Napster backlash. James goes into rehab and they hire an on-site therapist, and you know the rest of the story. St. Anger was not a good album from them at all, and I started to wonder if they had started the downward slide into obscurity. Don;t get me wrong, I know that they are still popular, very much so, but this new album is make or break time for them. They can still pull out of this tailspin, but it is going to be H-A-R-D for them to do so, considering their age now, and how much the Heavy Metal landscape has changed (and not for the better but thats another rant for another time).