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.
Cliché Storm: The story's a simpler retelling of the first game and it shows. The heroes, tired from losing battles against the Covenant, fight them for a map room that points them to another place. They fight some more, get to the other place, and find the bad guys are going to use an ancient construct to kill even more people with. So they blow it up and float in space. Also the Flood's there.
In the campaign on Legendary, Hunters are the 'bane' of your existence. Especially your Scorpion tanks. It gets even better that they tend to come fully upgraded.
Fridge Brilliance: Look closely on the visors of the SPARTANs on the cover, and you'll see the Arbiter and Elites. The Limited Edition cover shows the Arbiter and Elites. They're facing each other.
Good Bad Bugs: The unfinished Scarab can't kill units in cover, as hard as it tries. Put some Marines in the Covie guard tower in the middle of the map and you'll have the Scarab distracted for the rest of the level.
That One Level: Arguably every mission following Relic Interior can fall into this category on higher difficulties, namely Legendary. Often because of a frustrating gimmick the AI ruthlessly exploits.
Arcadia City forces you to play defense and offense as well as babysit blink-and-they-die transports. Every now and then a wave of fully upgraded Banshees bum rushes the transports. It's especially jarring when you lose them thanks to a kamikaze Banshee.
Arcadia Outskirts has you play defense against a lengthy Covenant Assault. A Covenant Mega Turret joins in the fray with an almost ungodly Rate of Fire, forcing you to divert units that would otherwise be fending off the swarms of Covenant Soldiers to take it out... and then watch them explode in-route from the said turret.
Dome of Light brings wave after wave of Covenant forces to your base to keep you from planting 5 mildly durable tanks. It's insanely difficult to protect the Hunter flooded base and still guard the tanks, even using leader transport powers tends to barely finish the mission.
'Scarab'. The titular Covenant walker tank acts as a stationary turret tearing through all your troops. If you take too long it's game over, and it can take a hell of a beating. To get the gold medal you almost have to glitch.
Anders' Signal. While the rest of the mission you can take as long as you like, the beginning part is absolutely nightmarish trying to keep your Elephant intact from the hard-to-kill Brute Choppers with nothing but un-upgraded basic squads.
The Flood. If you thought those space zombies were hard in single player, try fighting a whole army of them while trying to destroy their colonies to weaken their largest one. Did we mention that the colonies regenerate if you take too long to destroy them all?
Shield World. Yahtzee rage quitted on this mission for good reason. A race against the clock to rescue your platoons trapped behind Flood-infested lines of endlessly-spawning flyers and loads of artillery with little time in the beginning to build your forces before the first platoon gets overrun.
Reactor. Surprisingly not as intense in the beginning as the Covenant, Forerunner Sentinels, and the Flood are more interested in killing each other than you, but take too long and the Covies will churn out a Scarab to hunt your reactor down.
That One Sidequest: 'Running The Show' achievement requires a million points in multiplayer. This is reduced from the original three million. This represents literally hundreds of hours of play.