There are a lot of -blam!- ups in that game... But I believe the worst thing they did was remove the most iconic piece of equipment/AA in Halo. The bubble shield.
#Bringbackbubbleshield2013
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Back when Nak3dEli was playing Halo 3, he used a bunch of bubble shields to help with his Sierra 117 Zero Shot run. And I also had uses for them. Stopping those dumbass Floodgate Elites from spamming grenades after arming them with swords, saving my ass from 25 pissed off Hornets, and taking advantage of dumb Brute deployment by assassinating each one in that bubble or throwing in a spike grenade for maximum carnage. And of course, in MP, it was a common practice to lure players into it.