The Metallica debate, for me, can be summed up like this.
After AJFA, they realized they couldn't go anywhere different with thrash metal. They took it as progressive and technical that they could do with Justice (which is the reason its my favorite from them), and they also realized that by sticking to that high-speed, technical blitz, they'd do well, but not explode. Someone at the time, or perhaps it was within the band members, realized that if they got a warmer, fatter, less technical sound, they could be worldwide icons across both metal and hard rock. They saw the success other bands were having going that route (they absolutely saw the success GnR had with Appetite, and I have to imagine they saw Queensryche sell 3.5 million with Empire, since they had the same management company at the time, among others).
So, they did exactly what everyone accuses them of doing -- they "sold out," or more accurately, they simply "slowed and dumbed down" the music, and polished a bit. Took off some of the edge. In all honesty, I totally understand WHY they did it. And, like they knew it would, it worked in a huge way, and they became global icons and rich as one could possibly be. And they've sort of tried to navigate that ever since, trying to morph their sound into what's accepted at the time, to build on what became a multi-million dollar empire.
As a fan, I'm not mad at them, but realize the band I loved stopped with ...And Justice for All. Yeah, there are songs I love from all Metallica records, including St. Anger (even though it sounds like dog shit). But the vibe of the band is what I sort of refer to as "corporate hard rock/metal." It's all about the BRAND, as opposed to the BAND. Sure, they talk a good game, but when it comes down to it, I don't believe for a second anything they have done post-Justice has been purely artistic. It all has a business-bent to it.
Luckily for them, a lot of people love what they've done, artistically. I mean hell, most of post-Black album stuff doesn't even sound like the same band that did Puppets, but there are still some really good songs on it. Bleeding Me comes to mind immediately. And you give them credit for writing some really good songs after Justice (the Black album is OK, its just too polished for my tastes, for a thrash band).
And that last statement brings me to my final point, which I have said quite a few times over the years. Like it or not, when you name your band METALLICA, and you spend the first four albums of your career literally defining the sub-genre of thrash metal, seeking to play faster and heavier than everyone, you're pigeon-holed. Thrash bands, for me personally, have very little leeway in deviating from their sound. You expect a thrash band, well, to thrash. Heavy. Aggressive. All of that. Even Metallica's ballads were heavy through the Justice record.
That's why when I look at bands like Metallica, Megadeth, Testament, Death Angel, etc., when they get softer and less aggressive, I like them less. That doesn't mean some of it doesn't work. For example, I think Megadeth really sounded pretty damn good with Youthansia, but everything felt almost at the same tempo, which was a tick slower, and a bit of a bummer. Countdown to me, was acceptable, because the songs were still fast and heavy, just a little more polished. But after Youthanasia...Megadeth spiraled for a bit.
Simply put, for me, thrash bands need to always thrash. They don't have (again, this is all personal opinion on how I view music) the leeway of progressive metal bands like Dream Theater, Fates Warning, and the original lineup of Queensryche; or the other sub-genres to deviate greatly. I don't know why I feel that way. But thrash bands, particularly Metallica, who defined the genre, to me, need to stay heavy and aggressive. And when they don't, it raises eyebrows, big time.
And if you go back to 1991, looking at it in retrospect, coming off of Justice, no band in the world was as heavy as Metallica. Sure, you had Slayer, you had Anthrax, and other thrash bands and the death metal movement, and lots of stuff happening. But Metallica in 1988-1989 was the most bad ass band on the planet in terms of speed and brutality. I don't blame fans (who Metallica groomed up to the Black album as more family, and that they were "a part" of things) for revolting, and feeling betrayed -- they were. And some fans were ok with it, and accepted it. But a lot of the hardcore ones really had a hard time with it.
It was a normal reaction, but much, much bigger, because of Metallica's incredible rise to fame.
To a lesser extent, Queensryche did it with Empire. But then again, Queensryche was never playing brutality fast thrash metal, so while there was some push back, it was generally accepted. Metallica...not so much.
I still call myself a Metallica fan. But I always qualify it by saying "I'm a fan of the band's work through ...And Justice for All. After that, it is hit and miss for me." Because that's simply the truth. To deny the quality of some of their work after they stopped being a thrash band would be foolhardy, IMO. But the reaction of people to such an abrupt change is understandable.