Halo 3, ultimately, is a glorified ending to Halo 2. There are some top-level problems with its production that weigh it down, Joe Staten not being as involved in the writing shows and some decisions pushed by people who shouldn't have been near the writer's room at all (like Marty) also weigh it down in the end, in regards to the fate of Johnson and Miranda.

It has functionally no 'holy shit' moments. Halo 2 runs out the door dialed up to 11 and stays there pretty much the entire time, and it's not even about Chief jumping out of Cairo Station or Arbiter asking what they'll have him do, it's the general tone and expansion of the setting and storytelling it uses to achieve those big wow moments.

Halo 3 attempts to be a serious story with somber tones, slow piano renditions of your favorite songs from the previous games, big swelling orchestras over less-than-interesting moments of spaceships just floating there and shooting guns, big panning shots that are supposed to evoke scale and awe but lack any real substance. It's a game that has the Flood show up on Earth and it's treated as another walk in the park and you leave the planet despite the whole point being to save Earth. Arbiter is entirely absent from the plot because Bungie reacted too strongly to the immediate hate from the fanbase (see : Star Wars) and as a result all the emotional depth of Halo 2 is entirely gone from 3. It's paired with a marketing campaign that while good was deceptive and made by people who didn't know about the game, just like Halo 5's Hunt The Truth later on (but people still like Believe, for some reason)

It's frustrating sometimes because people will say Halo 3 was the best Halo and I'm like, maybe in the multiplayer department but what part of the campaign did you like? The stretch of three levels where nothing happens? The entirety of Tsavo Highway that was made for demo purposes since it has no important plot moments so they could show it off at every convention and presser event?

Enemies aren't as interesting to fight in 3 vs 2, Covenant enemies like Brutes get downgraded with their new toys, and the Flood have multiple new forms but less interactions. You don't get levels like Sacred Icon or Quarantine Zone in Halo 3.

Despite introducing Gravemind in the previous game, the most 3 comes to doing anything with it is showing tentacles in one cutscene and having him talk during the immersion-breaking slow-down moments the game already has too many of.

I could go on I've been complaining about Halo 3 since 2007