Why do people hate the St. Anger album so much?
I as a Metallica fan find it quite interesting.
To get a "correct" answer for this will involve a lot of work, but if one speculates wildly and freely, it could be due to the gap between people's expectations and what they got.
As I remember it St. Anger was touted as Metallica's return to the heavier stuff. People likely expected Master of Puppets, but got something quite different. The somewhat eccentric snare sound, and comparably low fi (for such a high profile recording) production did not help matters either. There were traces of nu metal in the music, something more conservative listeners likely did not appreciate.
Add to this the fan person rage amplifier which is the internet, and things can get ugly quite quickly.
I think they got more flak than they deserved for St. Anger, but that is outweighed by the praise received for Death Magnetic...