wanna be a shooter buddhistava, baby.
it might have been worth touching on colour design as well, something that seldom gets touched by members of the doom community because (a) everyone sucks too much to make their own textures and (b) poor colour choice is actually inherently forced upon doom mappers due to the limited doom palette.
a couple of schools of thinking on this one - the graphic designer in me says use three colours, no more, no less, to achieve striking, unified level design. Quake would be a good example of this sort of thinking. It also makes adding Focal Points easier (glowing red eyes to denote enemies that need shot, spurts of red blood to make things seem visceral, red crosses on medipaks... if you see red in quake, then you know it is important).
jaws, the movie, is another good example of this sort of thinking where all the props were carefully chosen to subliminally install a sense of dread in the viewer. the colour yellow in jaws only appears then the shark is hanging about (alex kitner's raft, the barrels quint uses to drag the shark to the surface). this theme, whilst subtle and unlikely to be noticed by the casual movie viewer, is repeated often enough for some primitive monkey part of our brain to associate yellow with danger.
note that the life jackets in jaws are not, as common sense might dictate, yellow; it would have utterly ruined the movie to have the main leads swimming off into the horizon whilst wearing the shark's signature colour, a subliminal suggestion that the shark was still out there, waiting beneath the water to eat our heroes and denying audience catharsis.
there is also the danger of making your level design static. Yes, human beings like symmetry, but we also appreciate movement and minor variations within. the bare bones of a level benefit from a level of symmetry, but making it an identical mirror image just makes things artificial. Consider a human face, the first thing human beings study as babies (thus patterning us to find symmetry reassuring and pleasing); yes, faces are mostly symmetrical, but it is with the flaws in that symmetry that we learn to identify individual faces (you have a mole on the nth quadrant of your cheekbone and a scar to your temple, your name is alex, you have a raging squint in your left eye and a birthmark in the shape of a burst tampon on your neck, your name is sophie). Again, you can condition a player to look for repeated symmetry in your levels and reward / punish them for it.
Consider a series of four square rooms, spread out of the course of a few levels; each time so far upon finding a square room imps have teleported in. You are, therefore, put on guard whenever you see a square room because you are expecting imps. So, by deliberately fucking with the player in square room #5 and blasting him with barons of hell, you are messing with the player's sense of symmetry, will put him on edge and throw out any previously ingrained notions he / she may of had.
expect the unexpected etc.
on that note, if you are struggling to maintain people's attention with your update videos why not get one of your minions to narrate a video for you on the subject they are currently concerned with? if people are bored with carnevil blathering then shove your texture artist out there and get him to mix things up a little.