Oh, and as I quite fancy saying a lot here, I'll go for a load of stuff that I keep in mind when mapping (even if I'm not all that good a mapper, and my opinions mean little

):
Gameplay:
-When I put in the monsters for my map, I always think about two seperate things. The first is how hard I want the map to be (Scars of the Wounded Prey is overkill on Revenants for that precise reason). The second is the impact I want the monsters to have. If I want the player to be running back out of the room asap with his guns blazing, he'll get a revenant in one ear, and another monster right in front of him. If I want the player to be plowing through the level recklessly, he'll be running at imps and shotgunguys, placed thickly enough to provide a challenge, but not so thick as to slow him down.
-When I'm putting in health and weapons, I tend to go for a theme. I experimented with limited weaponry in Scars, and acheived something different from your standard SSG slaughter map, IMO. With health, I tend to pick between having the player be stocked with health, and therefore able to be reckless and having health tucked away in the odd corner, making the player move more casually and be observant. Of course this sort of thing needs tuning, and I've probably frustrated people by limiting their health too much, or drowning them with medipacks before now.
-A highly important gameplay feature is traps. Everybody knows this, but few people know how to use them effectively. I have no fear in saying that I'm definitely not one of those who do. Seems I either have no traps (Warpzone), or far too many (Scars). However, I do know from playing experience, and my own building experience, that in between those two polar opposite examples, there is a level of perfection with traps. You need to decide how tense and jumpy you want your player, how hard the level wants to be and how long the map is. A really long map will only drag on if you stick in a plethora of multi-pronged traps. A really short map would benefit from such traps, however, as they'd add considerable length to a room which would otherwise be quickly forgotton. With ACS, a trap doesn't need to be a monster warp in, or a wall dropping down. You could move a whole wall load of detail into the floor and ceilings (as Ive experimented with), or you could fire projectiles at the player HeXen style. There are many things possible, like crushers, moving walls, lowering cages, and much more that I probably haven't even thought of. It's all in imagination.
-Giving the player an aim is useful. You can easily have a map that requires a couple of switches to be found, a key to be foun, a key door to be opened and then an exit switch and, provided you've made it well, that'd work. But wouldn't it be much more interesting if the player had to disable a security mechanism, infiltrate a demonic compound and detonate the icon of sins brain from behind? Once again, it's imagination and effort. One day I'll make an interesting map, and I'd lay money on it faring better than my other efforts thus far.
Architecture:
-The key to making a level (as far as looks and gameplay area are concerned) would be making what you WANT it to be. If you're making a high tech lab, but it only has flat walls and a low light level, you've not really done it, have you? If there's pipes filled with god-knows-what; chemical equipment; desks; experiments; machinery; bright, sterile conditions and a corpse or two in the corner that could have been a scientist you're talking. If your castle bares no resemblance to anything but a stone corridor, you've got work to do. (I'm drawing examples from Scourge now, as these are all examples of what I wanted, but didn't do). It should have carpets, torches, livery and the look of a castle. Think, if it's demonic, you want it to look evil. Stick in holes with flesh slithering past, have the faces of a thousand souls burning in agony behind grates on the wall.
-Details look nice, but don't necessarily mean pixel perfect images and stuff drawn in sectors. Having a pipe or two in a corridor could well make the difference. Having a border around lighting fixtures, and an intersting combination of floors in a room could well make it look good enough on their own. Stick something interesting in the wall, and you've got yourself a good look.
-90 degrees is easy to make, but will rarely look as good. I'm guilty of having too many square corners in a level, but I never claimed to be any good

Curve those square corners, and see the difference
-Symmetry is my biggest issue. Pretty much every room in every map I've ever made has some symmetry. Might be the lighting fixtures are all the same, or the basic layout of the room, or that it's just a rectangle. Ideas I've had for sorting this out involve making the walls look different (so it's slightly less oppresive) with computer panels and lighting fixtures, and having irregular shaped rooms where possible. I'm also toying around with making the detailing on my ceilings and floors be a bit more random, but I've had little success.
And yeah, that'll do
