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.
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: '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.
Arcadia and Dome of Light are also nightmarish. Arcadia forces you to play defense and offense as well as babysit blink-and-they-die transports. 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.
The level 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?