There's a lot to talk about here
Respawning of enemies (infinite or just one time reinforcements) is a mechanic that can be used to good effect but one has to find the right balance. Having no respawning isn't bad, but for long levels where the player has to backtrack much it does leave things feeling awfully empty, and rather predictable if you know enemies are only ahead of you. On the other extreme, infinite respawning can be very cancerous, and can leave the player (1) never having a free moment and (2) feeling bad about clearing rooms since it is ineffective in the long term.
A balance may be the most effective solution. Infinite respawning on large or closed maps can work (Redux G4 for instance, even though the compilers infinitely respawn (I think), the player still does get free moments and isn't hounded 24/7. The doors on Neither high nor low prevent the wasps from hunting the player down 24/7 so it isn't as cancerous there, either.) On the other hand, SLN,TLC, the "I just recharged after killing an infinitely spawned wasp, oh wait there's another) is more frustrating since it is open and generally hard to not have to nearly constantly deal with the things.
One time resupply of monsters (say, some baddies teleport in) is a good compromise, particularly if it is not used frequently. It is true that if it used frequently it could feel as if the designers are constantly trying to needle the player in the way infinite respawning does. However, if used infrequently enough, say, once every few levels, it could help keep things interesting and be an added challenge.
I mention the idea on Habe Quiddam because the winding architecture would allow the player to see the teleportation. Apart from showing off feature that Redux has that M1 doesn't, this allows the player to immediately mop up the newcomers if he doesn't want to deal with them later, whereas having monsters repopulate a room on the other side of a map allows for sneaky ambushes but could be more annoying. Maybe there could be a few levels where that would go well.
Perhaps another area where the player could see the respawning would be fire x100, the player reaches the control room and sees compiler teleport in that he will have to deal with later in the level.
I've become a little more appreciative of your concerns ever since I tried to work on a few modifications to Never Burn Money myself (plz don't ask to see, it is still in very early stages and I barely have time to do the commentary as is). I've found that some things that I thought would look good end up looking out of place (though maybe that's just because I am really bad at recreating M1 architecture). That being said, I have seen plenty of details/visual improvements (say, the exterior vistas of Redux's BGN) that do look appropriate for M1 and do make things look better. Visual improvements (even small scale ones that are just details) can look good but they need to be taken with a critical eye. If they don't fit the style or look out of place, they shouldn't be included. In this sense, you may be right that say, adding chairs, wouldn't help things at all, but since the point of this thread is to get ideas out there (even if many of them aren't good), I'm still going to suggest things and hopefully at least some of them will make the game better.
I also try to mention what purpose a section of the ship may serve to help let someone more talented than I am do the creative improvements.
Level 15: Neither High Nor Low
Tour of Duty Link: http://forums.bungie.org/story/?read=63728
I'll start out by mentioning the infinite respawning wasps, which gave me much pain on SLN, TLC. On this level, however, they have a habit of hanging out in the central room so they aren't nearly as annoying if you know to expect them in there. The doors prevent them from hunting the players down and leaving them no free moments.
There's a debate whether this should be a vacuum level. Here are some pros/cons that come to mind:
-It seems to be originally intended to be vacuum
-Marathon 1 needs more vacuum levels
-The level could use more spice
-What about the wasps? Wings don't work in vacuum.
-Has the player ever had a straight up fight against hunters (without vacuum or having to save BoBs or anything) I guess there are opportunities for this later.
Thinking about it, since the alien ship levels are probably not gonna be vacuum ones, this one probably should be just since it is the last chance (barring some strange things like making BOLFDD vacuum). In just the very next level we get a fair fight with hunters anyways.
As for the level itself, I can understand why Martin would later think that this is just a barrier. It's just one linear (albeit convoluted) path from start to end, which can ben entirely skipped if one "jumps" around the pillars in the stairs.
I think the best way to fix this level is to give the player something to do other than just "get to the end." If you've read the final terminal on Habe Quiddam, you'll read something-something-I-know-how-to-extend-the-range-of-the-Marathon's-teleporters-something-something. Shouldn't that take some hardware upgrades too? He says it's a "complex software enhancement," but it's not unreasonable to modify that slightly to a "complex enhancement."
After all, Defend THIS! is the only M1 repair level where you have to run around with a bunch of chips. Apart from giving the player something to do (finding chips that may not be placed in front of his nose), this may be a very convenient time to introduce some of Redux's boss monsters. This also circumvents the ability to skip the level with the staircase because the player will have to find uplink chips hidden around the map while still allowing for skipping of completed parts of the level if he falls down/goes back to recharge.
Of course, Durandal is still going to be as non-helpful as always and avoid talking about the mission while still talking about how the universe collapses in 15 billion years (which isn't even that much longer than the Earth has been alive, for an immortal AI it makes sense for it to be on his mind). Maybe he'll scold the player if he hasn't figured it out by the end.
Here's a fun fact, this is one of the few levels where the infravision is actually useful (since there are no compies)
As a final note: Time to make somebody pay!