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.
Base Breaker: Fans either loved the Arbiter and thought his perspective of the Covenant War brought a refreshing light to the Elites or hated him due to the fact that you were suddenly playing as the enemy and not Master Chief, without any hints that this would happen pre-release. Demoting him to the 2P character in Halo 3's co-op widened his status as a Base Breaker even further. Eventually, though, the hate died down as the uproar over Halo 2's misleading advertising campaign gradually faded away, with the Arbiter's popularity finally solidifying as the fanbase moving on to fighting about other things.
Best Level Ever: Metropolis. Drive a tank across a bridge fighting Ghosts, Wraiths, Banshees and Phantoms, followed by a battle in the city after a quick battle in a tunnel, and the level ends with the Scarab fight.
Even Better Sequel: It is debated on how well the story lives up to expectations, but this game is considered one of the best shooters of all time.
Franchise Original Sin: Dual Wielding. It is an interesting method of allowing players to mix and match weapons to accompany the situation, but the problem is nearly all the weapons you can dual wield with are inherently weaker than the single wield weapons (including the radical nerfing of the pistol) and thus require something to support it otherwise you have a serious disadvantage. In both this game and Halo 3 it works okay for the campaign but in multiplayer it was a well despised feature. It's telling that they eliminated the whole mechanic after Halo 3 and virtually no one has complained.
The exception was dual wielding a plasma pistol and SMG. Derisively dubbed - "The Noob Combo" by many, it allowed for players to instantly kill another player's shield with the plasma pistol and then quickly kill them with the SMG without having to switch weapons or wait for the pistol to cool down.
Game Breaker: Though not supposed to be something that players have access to, the Scarab Gun makes the level it can be found on completely trivial.
Nintendo Hard: Halo 2 is universally regarded as the toughest campaign to beat on Legendary, with Halo 1 coming a close second. Reasons why:
Player shields for some baffling reason are insanely weak, and go down in one hit. Another hit kills you. Coupled with the fact that enemies tend to fire from multiple directions and much faster, means it is very difficult to attack multiple enemies headon with having to retreat constantly.
In Co-Op, the Iron skull is turned on automatically and cannot be turned off. If one player dies, both get booted back to the last checkpoint.
Drones. Absolutely annoying on lower difficulties due to their swarm tactics and ability to fly about the level, here they are just insane. 15-20 Drones flying about, each firing a Plasma Pistol that will take shields down almost instantly. Have fun.
Jackals wielding Beam Rifles (of which there are loads and loads in several levels, Metropolis and Regret being notable examples) can kill the player in one hit, regardless of hit location. They also have insane accuracy, never missing their shots.
Brutes, once they become the mainstay enemies after the level Gravemind, are insanely resilient and coupled with their tendency to go berserk makes them difficult to deal with, especially when using Covenant weaponry. Their tendency to carry Brute Shots which can absolutely shred the player's shields with splash damage doesn't help.
Several levels force the player to use only Covenant weaponry, which makes them less than ideal for fighting the Flood or Brutes, the former being weak to human weaponry and the latter being easier to combat with certain human weapons.
Friendly AI, especially when driving vehicles, is borderline retarded and more often than not will drive the player directly into combat and then stop, leaving the player to get annihilated.
Porting Disaster: For starters, the PC version forces people to use Windows Vista or later to play it by checking to see if you have Direct X 10 installed, even though it was designed for the original Xbox which uses Direct X8.1. A hack soon popped up that disabled this check, and it proved to work just fine on XP. Other problems include that many keys are hardcoded to be bound to Games for Windows Live (screwing anybody who prefers a non-WASD keymap), network connectivity was patchy (GFWL again), and the modding toolset was crap (basically impossible to create new single-player content).
The Scrappy: Miranda Keyes to some extant, going in head first gets quite annoying at times. Especially if you are, well, surrounded by flood parasites.
The EU explains it a bit. Apparently she's trying to live up to her father's name.
Scrappy Mechanic: The rank system was surely the least secure aspect of ranked playlists, deranking was a notorious glitch in the system or result of a really skilled hacker. God bless the heart if anything more than twelve rank levels have disappeared off your record.
Uncanny Valley: In the remastered cutscenes, the Prophets have teeth that look very human. The effect is remarkably unsettling.