Halo: Combat Evolved shits on it from the highest mountain. Halo 3 had the most disappointing campaigns of the Bungie entrees. There were just so many things that were bad all around.
1) Brutes were just not fun to fight. Sorry, but Elites were night and day better. Their ability to use the world around them to their advantage -- especially in Combat Evolved -- is still unparalleled in the later Halo games. Everyone hypes the Brutes' use of equipment, but it devolved into nothing more than one throwing a bubble shield or a regen down while fighting. That was about as fun as it was in multiplayer -- i.e. absolutely fucking horrible. Occasionally, one would throw a flare down. Great, I just got blinded by an AI. Fun! Their AI and tactics were rudimentary. They had no personality, at all. They were just bland, straight down to popping their armor with a few shots and sending them into the predictable rage mode. They were better than in Halo 2, but that's not saying a whole lot.
2) The story was fucking awful. Remember that epic cutscene at the end of the game? Bungie promising FINALLY, we will have the invasion of Earth that was promised in Halo 2? All that "glorious" battle happens off-screen. We're left with yet another couple missions through Earth and then jump back into a ship and go across the galaxy -- again. Something, something, new Halo ring being built, must destroy Ark. Cortana's rampant state and her "deal" with Gravemind at the end of Halo 2 are glossed over and dropped. The Gravemind itself is essentially an afterthought, aside from those awful moments in The Storm, where your player slowed to an absolute crawl while voices played through the game. Real good idea with that one.
3) The character assassination was unforgivable. The Arbiter is the single BEST character in the series. He's better than Chief, he's better than Cortana, he's better than the walking stereotype that is Sgt. Johnson. Yet he's relegated to a one liner to start the game, a few choice comments throughout the campaign and a cutscene at the end. What happened to his arc that began in Halo 2? He was a fleshed out, believable and sympathetic character relegated to the token sidekick in Halo 3.
The Prophet of Truth was even worse. For one, you swap out the amazing voice acting of Michael Wincott in Halo 2 and replace him with the most phoned in performance from Terrance Stamp -- eclipsed only seven years later by Peter Dinklage in Destiny. The Prophet goes from being a conniving, manipulative character who may or may not believe the religious babble he's spouting in Halo 2, to being a complete raving religious zealot loon in Halo 3, only to be killed in the most anti-climatic way.
4. Cortana. Do we really need to say anything else about this level? People love to shit on Library in Halo: Combat Evolved or sniper alley in Outskirts in Halo 2, but NONE of that reaches the stupendously awful levels that is Cortana. This is the 10th circle of hell, just without the frozen wastes. Let's design a level with the ugliest art throughout the entire game -- which is really bad considering Crow's Nest and the awful human environments are ever present. Let's turn this into one giant circle of death and offer few landmarks to actually guide players through it. Let's then populate it with Flood. And not just the mildly annoying type that make some of us reminisce of Serious Sam. No, let's add in juggernauts and little fuckers that crawl on the walls and ceilings and shoot barbs at us. And let's take a page out of Call of Duty's playbook and add in infinite spawning. No. No game, no matter how high the highs are, can ever be considered the best in its series when it has a level like this.
5. The Scarab battles are vastly overrated. Granted, they are better than the travesty of Halo 2, but that's not saying a whole lot. Sure, they look really fucking cool when you first engage one. You've got marines flying around on Mongeese shooting rockets, you've got Covenant on Ghosts zipping about and you've got this giant fucking robot you somehow have to bring down. And then after that five seconds of awe passes, you realize how ridiculously simple it is to take them out. Shoot the legs, climb on board, shoot the ass, it's over. That's it. That's the most shallow "boss" fight I'd played up to some of the garbage in "Destiny." At least they had the common sense back then not to fill it with a ridiculous amount of hit points.
I could go on. There's the awful Ghost and Mongeese run at the end, which tries to pay homage to the superior Maw run in Combat Evolved, but just becomes more of an exercise in frustration and futility. There's the awful waste of space that is Johnson's character -- including that cringe-worthy ending and "boss fight" with Guilty Spark. There's the HORRIBLE character models -- looking at you Lord Hood -- that are somehow worse looking than Halo 2. There's nothing in the game that has the highs of battling through Assault on the Control Room or attacking the beach in Silent Cartographer in Combat Evolved. There's none of the awe or mystery of Halo in Combat Evolved. There's not even the big story payoff promised in Halo 2. It's just not good. It was three years of hype built up to this amazing payoff that fell flatter than The Dark Knight Rises.