The Canon disruption was huge...There was an established story (A remarkably well crafted one at that) that had been established for almost 10 years and Bungie threw it out the window with the air of "Our game, our story." When in reality that is not the way things are done.

The next big thing was that Reach set the stage for what in my opinion is the downfall of Halo's multiplayer: Armor abilities... What always made Halo great for me previously was the idea that everyone spawn with the same weapons, the same abilities, the same everything. Victory was based on skill, team work, and map control. Reach introduced this odd Rock Paper Scissors mechanic where slight advantages dictated by what armor ability you had affected the out come.

Then Halo 4 doubled down on this premise and made it even worse.

That said thoough despite its flaws imo I spent countless hours playing Reach (Less than Halo 3 mind you but a hell of a lot regardless)...GREAT GAME!!