I'm likely to get a lot of shit for this rant/opinion, but fuck it:
I remember when I was young and stupid (the heaviest stuff I had listened to was like, Korn or early Marilyn Manson at the time) and when I first heard Mandatory Suicide, it scared the shit out of me. Then the shock wore off and it was "pretty cool", so I checked them out some more, but I wasn't 100% impressed with Christ Illusion the same way I was with Hell Awaits, and here's why:
Compared to the speed of Show No Mercy, the aggression of Hell Awaits, and the slower tempos of South of Heaven, I just think that World Painted Blood and Christ Illusion just fall behind slightly. They're great, but not...great
. I like those albums, don't get me wrong, but if all of Slayer's output was comparable to a track meet in high school, the 80's and early 90's material would be the elite, cream of the crop, the shit a lot of people love to talk about. Their later output would be the runoff of that, great, but not "100% Badass Hardcore Radikult"...and Diabolus in Musica would be the sorta chubby kid that only got to run because the others felt bad for him.
TL;DR: I think most of the hate comes from the opinion that Slayer hasn't changed their sound much since the mid-90's compared to their 80's output.