A few weeks back someone asked "when did Metallica start to suck".
First, I don't think Metallica sucks. They are still miles better than most of the so called metal out there today.
Unfortunately, their current state is so diminished from when they were the best metal band in the world; that they suck in comparison.
Metallica doesn't suck; they disappoint. They've long since jumped the shark.
Ok, but when, and why?
I'd say Metallica really jumped the shark in the long period between "Metallica" and "Load" (ReLoad is essentially the second half of the Load double album).
Although many hardcore metalheads hated Metallica Metallica (the black album), I think it remains today the single best example of metal crossing over to a wider audience; and that isn't a bad thing. Metallica was still hard and heavy, but made the music somewhat more accessible to those who were not into the hardcore classical music based speedmetal, or punk based thrashmetal that Metallica had blended so well in previous albums.
I would say that they peaked with "Master of Puppets", which I still consider the greatest metal album of all time. Other than Damage Inc. every single track on that album is brilliant; a perfect example of Metallicas thrash/speed metal fusion (Damage inc. isn't a bad song, but it is more appropriate to an earlier era of Metallicas music. It's a throwaway almost garageband thrash track, and the album would have been better closing with "Orion").
Metallica then plateaued with "And Justice for All" and "Metallica". They were both musically and artistically excellent, but they did not quite reach the height of "Master...", nor were they an artistic stretch for the band. Both were exercises in refining their sound, and their technical abilities rather than reaching for new heights or breaking new ground.
You may attribute this change to the loss of Cliff Burton, but I think it was simply that the band found the peak of their possible achievements in "Master" and simply could do no better. Every band finds their peak, and declines (or breaks up, or both) it's the nature of the beast.
Now that I've said that, I'm going to almost contradict myself, by saying that it WAS Cliffs death that changed the band irrevocably.
The first major consequence was that Cliffs loss initiated a MAJOR change in James Hetfield... simply put, he lost his art. He had his anger and his pain, but Cliffs death seemed to have take the artistic passion out of him, and replaced it with a towering resentment for Lars, who to this day seems oddly false when discussing how Cliffs death effected him.
In fact, if you look at everything since, it seems clear that Hetfield is happiest, when he's reliving the early days of the band, as in Garage Inc.
You have to remember that Metallica is essentially two bands. James Hetfield and then later Jason Newsted, are thrashmetal punks to their core; whereas Lars Ulrich and Kirk Hammett are quintessentially speed metal.
This tension between artistic elements is what made their music great, and Cliff Burton who was a punk at heart, but was also a great lover of Jazz and Classical music (and speedmetal, which grew out of classical influences); acted as the bridge between these two elements in tension (This can be best heard in "To live is to die" and "(Anastehsia) pulling teeth").
Without that bridge, the magic mix was lost, and it was simply a tug of war between James and Lars.
"Master..." was the last album where those elements of speed metal and thrash metal were truly blended properly. "Justice..." was an exploration of the excesses of speed metal; complicated by the gigantic ego of Lars Ulrich (and corresponding anger of Hetfield) making the production of the album a sloppy, poorly mixed nightmare.
Bringing in Bob Rock on "Metallica" was really an attempt by the band to put that bridge back in place, and restore some of that "mix" lost when Cliff died; and to an extent it worked. Combined with the far more professional production that Rock provided, and his ability to mediate between Hetfield and Ulrich (mostly by making them hate him wore than each other), "Metallica" was the most balanced production the band had yet made.
Of course, that's the problem with it.
Metallica wasn't great because they BALANCED thrash and speed metal, they were great because they wildly rollercoastered around the entirety of thrash and speedmetal, even within the same song.
Bob Rock gave them a better produced, better packaged, GOOD album, but he also muted the excesses.... the ups and downs that make great art. There was no more rollercoaster... more like a freight train, running down a straight line at 90mph.
Make no mistake "Metallica" is a very good album, and has a couple of really GREAT songs on it... but it is... unmagical? I don't want to say uninspired, because the very personal lyrical content that Hetfield included is in many ways beautiful, painful, and entirely revealing... but it just doesn't have the spark that the first three albums did.
"Metallica" also marks the last significant artistic contributions that Kirk Hammett or Lars Ulrich would make to the music of the band.
The entire post "Metallica" discography is essentially Hetfields artistic choice, fueled mostly by his anger, resentment, and disillusionment. It has also moved deeper and deeper into a grunge and groove metal mode, without the classical speedmetal influences; and only occasionally breaking out into the hardcore thrash that Hetfields artistry and passion is rooted in.
Honestly, I think the last time Metallica did something great, it was on "Garage Inc." where for a few minutes James clearly felt like he was expressing himself fully, even though it was through other peoples music; and he and Lars were able to actually ENJOY playing with each other again.
Load... well, there is some excellent songwriting in load, and a few quite good songs, but overall it's missing inspiration. There's a lot of anger there, but no genius, no life to it. In Load, Metallica essentially abandoned their roots in punk, thrash, and speedmetal; and moved into a more hard rock, blues oriented structure. It's a simpler, more accessible, easier to play, write, produce, and sell music... but it just isn't as good.
It isn't really Metallica, it's almost numetallica.
St. Anger is just more of the same, only... angrier.
I've heard death magnetic, and I agree it's heavier than Metallica has been in years, but again there's no feeling to it other than anger and resentment.
Honestly, I think Hetfield should break up the band and either form a new thrashmetal band, or do some solo work; because this stuff is obviously not satisfying him...
...though really Metallicas artistic output since 1991 can best be thought of as James Hetfield with the support of Kirk Hammett and Lars Ulrich, rather than as Metallica (Newsted was ignored entirely for Load, and Trujillo has very little artistic input at all, though he is certainly a very strong bassist, and writes his own parts).