The problem I've always had with X-plays rating system is the x/5 system. not that any system is really good, cause its all an opinion really, but people generally look at an x/5 in one of five (six if you want to count a 0) ways:
0/5 - 0% nothing redeeming
1/5 - 20% still really bad
2/5 - 40% eh, not as bad and its half way decent I guess, maybe a rental
3/5 - 60% above the 50% failing grade, justifiably buyable
4/5 - 80% alright, we're getting to the good stuff, it might not be top 1% or anything, but still great game
5/5 - 100% no reason not to buy, no flaws with the game (otherwise it wouldnt get a 100%)
and there's just not a lot of in between ground. People will rattle off the star score and say "dude, it got 4/5 stars, its great" when it can be just a really low 80%, or "man, it got a 5/5 this game is perfect, why wouldnt you want it?"
then you compare it to other review sites where they use a full 0-100% system and people get pissed.
Plus I think IGN was a bit overzealous giving out that 100%. I've never seen a game that was perfect that didnt leave something to be desired or you didnt find something wrong with.
I dont know TL;DR, the people who are pissed are probably having a fanboy period, especially after seeing the IGN score, but I trust Adam in his judgment. Hes sold me on a few good games that I would have probably over looked had he not. in the end though, number systems are flawed (oh I know, wow! what ground shattering observation! /sarcasm) and you should always just read/listen to the comments on the game and see if you agree with them. Im sure not EVERYONE thought aquaman or drake of the 99 dragons was complete and total garbage (... maybe).