Finally had time to read through most of them and the reviews seem actually really great so I don't know what people are freaking out over and were expecting. The Halo series as a whole doesn't have a great storyline and 4 was by far the worst. The gameplay is what everyone comes to these games for and after the stepback that was 4 every review I have read is just praising the changes to the game. You have to also remember I doubt any of the reviewers played the game in 4 player co-op which is going to dramatically change how much fun your having being that the game is completely built around that.
The issue for some is need for Halo to be over 90% MC and "defend" status of XB1 and deliver a return to Halo glory days as enormous selling title.
At this point for the franchise and given current market these were lofty goals that were bound to leave feeling dissatisfied they weren't met.
The EU and the over-extension of what was a fun but cliche SF backdrop to exploring new places and killing evil aliens has resulted in an overly convoluted and increasingly dull SP narrative with characters and plots that cannot take the weight of expectation placed on them.
With CoD dominating online MP for so long, Destiny more recently and the disappointment of Halo 4 MP the MP (which looks to have delivered from reviews) was never (no matter how good) going to stand out the way Halo MP on console used to. Toouch competition and FPS MPon console is too established now for one title to stand out the way Halo used to.
Finally critical reviews for big franchises seem to always have a few pulling average down meaning beloved franchise can easily dip below the coveted 90%.
In short while apparently a great MP game with a solid SP game and a decent to poor narrative depending on your EU knowledge the game was never going to be like Halo 2 and Halo 3 in terms of standing out as a beacon for the platform.
And for some that's too much to take given they're platform is trailing globally.
Man though the Tomv Raider thread is going to be worse: guaranteed.