* AntiClimaxBoss: [[spoiler: Guilty Spark.]]
* BestBossEver: The dual Scarab battle on The Covenant. Probably the coolest battle in the series.
* BestLevelEver: Most players would nominate the entire second half of the game for this- with the notable exception of ''[[ThatOneLevel Cortana]]''.
** Then "The Covenant", where Master Chief and a small army takes out two Scarabs. And there are not cutscenes for that, you have to pilot something onto the backs of those things and blow them up yourself.
** The Storm is the best level in the first half.
* CompleteMonster: Truth is far beyond redemption by this game, being quite willing to wipe out all life in the galaxy just to be viewed as a god.
* CrowningMomentOfAwesome: The level "The Covenant", where you get an amazing shot of a battlefield you're about to be in (complete with two Scarabs and a whole lotta Banshees!) while having a lovely Crowning Music of Awesome.
* CrowningMomentOfFunny: The Easter Egg from versus play when you have flipped over an Elephant. "Hold RB to flip... [[DelayedReaction wait, what??]] How did you do that???"
** Something about the three-part MoodWhiplash between Cortana's monologue about how she chose John because he was luckier than the other Spartans... followed by [[ItsRainingMen John falling from the sky like a shooting star]] and plowing into the ground... followed immediately by [[MostWonderfulSound Achievement Unlocked.]] [[MundaneMadeAwesome An achievement for starting the game.]]
* CrowningMomentOfHeartwarming: In [[ThatOneLevel Cortana]]:
-->'''Master Chief''': You know me. When I make a promise...
-->'''Cortana''':...you...keep it. I ''do'' know how to pick 'em.
* FridgeBrilliance: Cortana says the reason she chose Master Chief over every other Spartan is luck. Check out what happens to Noble Six, a SPARTAN who is as equally skilled as John, in ''VideoGame/HaloReach'', and her statement reverberates in hindsight.
** The policy of the military is that [=SPARTANs=] never die, they are just listed as MIA. That's exactly what happens to Chief in the most literal sense.
* FridgeHorror: [[spoiler: If Gravemind [[VillainBall wasn't in such a hurry to get to the Ark,]] the Flood would have had plenty of opportunity to infest Earth and wipe out what little remains of humanity, leaving the Chief and company backing up a graveyard while they were preoccupied with Truth.]]
** Well, [[spoiler: they did try to, dropping off an infected Covenant cruiser in Voi, which would have quickly taken over the entire planet had it not been for Half-Jaw glassing the immediate area before anything could escape.]]
* HarsherInHindsight: One of Sgt. Johnson's last lines is even worse after VideoGame/{{Halo 4}}.
-->'''Sgt. Johnson''': "Don't - don't let her go. Don't... ''ever'' let her go."
* {{Mondegreen}}: Quite a lot of people thought the Arbiter's PreMortemOneLiner was "[[PrecisionFStrike Asshole]][[note]]Actually "And so"[[/note]], you must be silenced!"
* MemeticBadass: In ''Halo 3'' multiplayer, [[LethalJokeItem the Cone]]. In the first days of release of Halo 3, a player shot the energy barrels of the level, [[http://www.youtube.com/watch?v=ym0BtwmCvoc and was subsequently killed by a flying traffic cone]]. [[SuperSoldier SPARTAN-II]] are afraid to tread near them.
** Water, [[SuperDrowningSkills bane of Spartans]].
* MoralEventHorizon: [[spoiler: Spark crosses it when he kills Johnson.]]
* NightmareFuel: Contrasting the previous games, now the Marines (and any Covenant, really) can be [[FaceFullOfAlienWingWong "Floodiefied"]] in real-time. Yes, they turn them into Flood in your face, with a touch of BodyHorror.
* PlayerPunch: [[spoiler:The deaths of Keyes and Johnson. Cortana's MindRape by the Gravemind]].
* TearJerker: Many.
** [[spoiler: Johnson's death.]]
--->'''[[spoiler: Sgt. Johnson]]:''' Send me out... with a bang.
** [[spoiler: Miranda Keyes' death.]]
** [[spoiler: The Arbiter's refusal to believe that Master Chief is truly dead.]]
** The final line of the trilogy: [[spoiler:"Wake me... when you need me."]]
* ThatOneLevel: For many, Cortana, particularly if on higher difficulties and god help you if you're doing it alone. If there's anything worse than Flood rushes plus endless needle death spamming your shields from Flood turrets beyond easy range of your weapons plus the walls being full of Infection Form pods that will pop open at a single stray shot, and thus re-animate all the Flood you've already killed behind you...
** The real reason "Cortana" is hell, aside from having the same problems as The Library: When you are playing the level, both Cortana and Gravemind like to have moments, where everything slows down as you hear them talk. The dialogue just turns the entire level into one giant Mind Screw.
* VillainDecay: The Prophet of Truth goes from a MagnificentBastard to.. [[{{Expy}} Well, the]] [[SmugSnake Prophet of Regret]].