Here's what I think makes a good DM map:
#1 - GAMEPLAY.
This is a tall order to achieve because you can't exactly define it. But here's some things to achieve it.
a. Good flow. Each area has at least two ways to get to it. No dead ends, unless the dead end is to purposely make it hard to go and get something like a shield belt.
b. Smooth floors and walls. Meaning when I am dodging around or running backwards, I don't want the walls to have a bunch of square edges where I get caught on, and I don't want a bunch of junk on the floor preventing me from dodging. Absolutely you can have decorational stuff, but if it is in the playing area, make it smooth.
c. Balanced Weapon/pickup placement. Spread the stuff around evenly.
d. Minimal extra pickups (amp, invis, health vials, etc) - don't put every single one of these in the map. Then the map is more about the pickups than the skill. Don't overdo the health, the game is called Death Match, not Survival Match.
e. No Teleporters! These destroy the feel and flow of a map (hey, I did title this post "IMO").
f. Ceilings high enough so you can jump without hitting your head.
#2 - VISUAL.
Well for me if you nailed the gameplay, it can be ugy as heck but still fun. But here's what I look for visually.
a. The textures have to look half-decent and work together. And they should be aligned.
b. Asymmetrical geometry. I don't like maps where someone just mirrors half of it to make it twice the size. I just get bored of it twice as quick! And I really can't stand Morbias][ for example, it's just a circle, it has no variety. Boring. So to me, a map that doesn't have a single area repeat itself is top notch.
That's it for me. And a minimum requirement: Path the bots! Oh I didn't mention Z-Axis, that is not too important for me, but it is a good way to add variety to the geometry.
I'd like to hear your opinions too on what make a good map.
I like maps with booby traps. So if you're not careful, the map will kill you long before other players do.