These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
YMMV: Halo Combat Evolved
Best Level Ever: Silent Cartographer, Assault on the Control Room and The Maw are probably the best ones, though the only really bad level is The Library.
Fanon: The second SPARTAN in campaign co-op is generally believed to be Noble Six by fans, had he escaped Reach. Before that, the consensus was that it was Linda-058, who canonically was in cryo on board the Pillar of Autumn during the game.
Really, it's probably still the most accepted fanon by people who pay attention to the Expanded Universe, particularly after her cameo in the remake.
Game Breaker: Punch Packing Pistol. The pistol is more powerful than the sniper rifle in practical usage. And all because someone at Bungie accidently slid the damage power up immediately before the game was sent to production.
One of the first things stated about the Aniversary remake is that the pistol will remain completely unchanged in power and accuracy. And boy, is that ever true.
The bandana skull from Anniversary tears the game to ribbons. While there's the obvious problem with shotguns, Sniper rifles and rocket launchers, the real breaker is infinite grenades. In a game filled to the brim with tight spaces and walkways the player can chuck grenades like mad with little need to aim. Add Grunt Funeral for extra power. Most of the achievements are still attainable with it too. In fact, you can use it to make one of the achievments (which requires you to beat a level on Heroic with three skulls on) a sinch.
The PC/Mac port made the Banshee available in multiplayer, with its Fuel Rod Gun intact. The latter is what particularly makes it a Game Breaker, due to its unlimited ammo, being fired from a high vantage point, flipping ground vehicles with ease, and just plain wasting infantry. It does have a somewhat slow rate of fire, but one well-aimed shot is generally all it takes to ruin someone's day.
Hilarious in Hindsight: Considering just how much the EU expanded, and how far the games went, the final words of the campaign:
Cortana: Halo. It's finished.
Master Chief: No, I think we're just getting started.
Iron Woobie: Guilty Spark, according to the Terminals. He waited for 100,000 years alone on his ring, refusing to leave despite his loneliness and risk that the Flood might break out or that his ring might fall into disarray. He went crazy.
It Was His Sled: The Flood appear in the level "343 Guilty Spark". It was a massive twist at the time, but now it's one of the game's more notable moments.
Lady Mondegreen: Combat Evolved introduced to us "Dustin Echoes", after a line where Cortana responds that all that's left is "just dust and echoes." Bungie even joked about this on their commentary for the cutscenes!
The Library is long, repetitive, full of Flood, dark and you can lose your way easily. Plus you have to deal with Guilty Spark for the whole level.
The indoor segments of "Assault on the Control Room" have you go through several long identical looking hallways. Not only is it boring, but it is very easy to get turned around and go back the way you came.
The indoor segments of "Two Betrayals", which is "Assault on the Control Room" going in the other direction.
The above two are fine if you know how to read the floor arrows. What takes the cake in "Two Betrayals" is Pulse Generator 2. Up to four rocket-wielding combat forms can be among the 20 that jump you just as you're making the required shield overload. The nearest health pack is all the way at the bottom of the canyon outside, so have fun reverting if you've anything less than full health.
The last section of "The Maw" requires you to drive a Warthog along an obstacle course against the clock. If you find the Warthog hard to control, this is rather annoying. And when you finally reach the target, the pickup transport for which you have been waiting crashes, and you have to reach another target, within the original time limit, which may result in having to start all over again. Which is also rather annoying.
Multiplayer has Chiron TL-34. The sheer number of teleporters (which would be phased out by Halo 3 due to camping concerns) led to a super-confusing map that has virtually no gameplay flow. Bungie acknowledged this and teased a "Chiron TL-35" during Halo 3 as a joke.