Looks like we can agree on this.

: so I thought you were referring to your reply to what I said, which was

Alright, so you are going to pretend that those quotes were my entire argument.
Let's go to some of the stuff I said that you never replied to.

Why the Didact should have known that the Chief was carrying a nuke:

Luminaries.
They're these nifty little sensors that can locate humans and discern an advanced AI from all the other electronics on Harvest, and they can do this from across a solar system.
And these are the inferior Covenant-built copies.

So yes. The defenses were firing back at the Chief when he was doing the trench run. If Forerunner technology is all it's cracked up to be, we can assume that the defenses were scanning the Chief. If we go by Greg Bear's novels, we can assume that ancilla in Didact's ship and Collector armor were keeping him up to speed on the UNSC fighter blasting through the ship's pathetic excuse for defense turrets.

And once the Chief was on the ship, and the Promethean Knights were reporting that there was boarding action, the Didact would have scanned the Chief, if only to figure out what witchcraft his wife did to make the Chief immune to the Composer.

Why the Didact is an idiot for monologuing when the Chief got close:

Sigh.
Ok, fine, so I only watched that cutscene once. Whether the Chief was sprinting, walking, or breakdancing toward the Didact, the fact remains that he was carrying a nuke the whole way. Didact's reaction is not "Get that thing away from the Composer", but "Hold the Chief up and launch into yet another cliched monologue". Don't get me wrong, a good villainous monologue can be truly a thing to behold. When it's not... well, it can make the villain look like an idiot. Especially when he forgets some very basic precautions.

Maxim #8: Mockery and derision have their place. Usually, it's on the far side of the airlock.

: if you meant for something else to show up there, I can't see it, and if you
: were referring to something else, it wasn't clear to me

: I have difficulty seeing how could be done properly and be fun and give the
: impression the Didact had done all he could. Why not drop the whole floor?

Just out of curiosity, how long's it been since you played the last level of Halo 3?

Notice how some areas took longer to self-destruct than others? Or how there were entire safe zones?
Some areas will be be impossible for the Didact to drop out simply because they're integral to the structure or function of Requiem. That's your destination.

: to feel like they've earned what, exactly?

Make it so that, when they have fought their way to the Didact and killed him, they actually earned their victory. Make them feel that the Didact was a Forerunner with an army and a reliquary at his disposal, not a half-baked mashup of Voldemort and Shinzon.

In addition, this means no quick-time events.

: bigger guns only do so much-- case in point, Scarabs

Even the Chief had to run when the Covenant started glassing.
And hey, bigger guns would have been a welcome change from fighting Prometheans and pushing buttons.

: I haven't the faintest idea, just like how I don't know how the Composer
: itself works. I'm OK with that as long as they don't explain it with
: ridiculousness like they did with "hard light" and as long as
: how either of them works doesn't become a plot point (and probably a deus
: ex machina) before we're let in on it.

Fire control.
My theory is that the Librarian's modifications changed the Chief enough that the Composer didn't recognize him as Human. While the scanning process itself was hazardous to human health, the Chief was spared from the full deconstruction.

I prefer this theory because the Composer disintegrated human flesh, bone, and even the clothes those scientists were wearing. "Genetic modifications" making someone immune to that makes about as much sense to me as an MRI making someone immune to bullets.

: Oh I understand, that's what I did too, somewhat, but no one was forcing you
: to stop playing.

No, nobody was forcing you. You just had to do it in case it was actually tied in with the plot.

: No, I'm going to gut you with a batleth, you ptaq.

That would be a lot more intimidating if I didn't know what a batleth is. Do your worst! >=D