Sniping/Camping
Most people felt that sniping was part of the game and couldn't be designed away. Camping is not the same thing as sniping, where sniping takes some skill and camping is based on plain luck and bad level design. One of sixteen thought that special sniping locations should be designed, others promoting sniping felt that snipers find places to snipe from anyway. Only one reply was specifically anti-sniper. "Another player should not be upset because he can't find the sniper, this is war..."

Traps
A majority of the replies didn't want traps as part of the game, mostly because traps doesn't award frags to a player. Those who wanted traps wanted them to be designed as an obstacle when trying to get a powerful powerup or weapon. Narrow ledges above lava and such. "... the idea is to fight the players, not the map itself."

Lighting
This one is the hardest and the easiest to sum up; a well balanced mix of the both. Hmm... If I'm to extract anything from the replies to this question it's that most wanted a somewhat lit level, at least where the fighting takes place. Dark places is justifiable if not overdone. Almost no one wanted blinking/pulsating lights (only because of the slowdown in the engine). "... DM isn't really much fun if you can't ever see who you're shooting at."

Water
Nobody wanted big water areas. Water was ok if it fit into the look of the level itself or was used as a way of making it harder to get to the goodies. The major opinion was that it slows down game play. "The occasional pool of water that cleverly connects two rooms might be alright."

Weapons balance
Give me the rocket launcher! Only one out of sixteen felt that the RL was a bit to powerful in a DM level but admitted that it could be included if made hard to get to. Almost everyone wanted more than one RL, spread out through the level. Another few felt that all weapons should be represented more than once. This was linked with how big the level was though, with a bigger level needing more weapons of course. The majority of the replies wanted all weapons at least on one location. Those who uses the double barrel shotgun should be able to play the level even though I as the designer don't like that particular weapon. "2 RL's would be nice, levels with only one RL are pretty lame..."

Colosseum or the sewers
Another hard one to pinpoint an answer to. Again everyone wanted a healthy mix of both, not to cramped though. Some felt that arenas made it to easy to just fire an RL into any group of people and rack up the frags. If designing big areas, one would have to make sure that there's enough obstacles to promote some skills. The element of chasing another player down dark, winding corridors was appreciated by some but they felt that most of the time it boiled down to using the RL to clean out these corridors without to much effort... "Give the player some room to maneuver for Christ's sakes."

2 or 32
Very target specific. To satisfy both 1 on 1, LAN, Clan and Quakeworld battles is almost impossible. Design for a specific group and stick to it. "This is an intended-audience question."

Respawn points
Out of the way from snipers, not to far away from some minor upgrade (weapon or armor) and not directly in the heat of the battle. "If it's a crowded level, you would want to get stuff ASAP before you die."

Textures/environment
Once again we see two camps, one feeling that the environmental should be close to reality, thus making the player more immersed in the surroundings. The other side feels that a DM level should be designed for DM, even if it makes for some non-realistic designs. Everyone agreed on the fact that textures did matter. No matter how intelligent a level was designed, if its ugly, no one will play it. A coherent theme is very important, let it be space stations or medieval castles. Stick to it! Some pointed out the importance of using Quake's true 3D capabilities and the importance of designing in all directions. Don't overdo with doors that has to be opened or things sticking out from the walls, DM is about action. "Finally, good texture placement just looks good."

Powerups
Very dependant on level size, most felt that the really good stuff should be either very hard to get to or not included. A Quad is only useful if the other players has a way of getting on a somewhat equal level, say with a RL. If to many powerups are stored to close together or to close to respawn points the balance is lost. A lot felt that the Red Armor was to powerful as well as the Pentagram. Ring of Invisibility was another one that was to be used sparingly. Some felt that to much health was bad for gameplay and lead to people running away to heal up as soon as they got hit. "Quad is great when you have it, but sucks when your on the other end."

Wrapping it all up
Woah... I don't know if I can do it neatly. Even though we only got 16 replies, most of them were very enthusiastic, spanning several pages when printed, making it hard to summarize things. I was hoping on giving you some statistics but because of the nature of the replies it's almost impossible to turn them into numbers. If anyone is interested in seeing these replies, let me know and I'll forward them to you.
As mentioned, I'll see if I get the time designing a level with these suggestions implemented. If not, rest assured that I will use them in any of my other coming projects... ;)