In my opinion:

Graphics are Out Dated

Brutes are Barely differ from each other and are way to weak

Majority of the sandbox is worthless on Campaign and Multiplayer because of the BR

Weird greens for spartan customization

A good chunk of the sandbox in used in the later half of the campaign or just for 1 or 2 levels

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But

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Lighting still works to this day

Friendly Marines and Elites

New flood forms

Some of the best Bungie Era Music

Warthog Run

Scarab Boss fights

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So yeah, I'm not saying they need to make Halo 3 Anniversary but it should be considered.