From my efforts, I've decided that I have little talent for designing good architecture from scratch. If you can make a map, then get a few people to playtest it and comment, then go back and fix things up accordingly, test again, etc. you will begin to beat the thing into shape. At least, that's how I have to work.
In the end, nothing beats "good gameplay", whether it's solo play or deathmatch. Eye candy is always appreciated, as well as perfect texture alignments, but if the gameplay isn't there, your efforts are wasted. How to define good gameplay - I don't know, other than having an acceptable balance of threat, offensive supplies, places for cover and chances to recover health. Some ebb and flow, not just continuous battling.
For deathmatch, you don't usually want a map which forces the players into fixed paths, there should often be a choice of directions to approach map areas so the element of surprise and tactics can be a part of the game. The best goodies like energy weapons can be objects to fight over, putting players at risk if they choose to go after the goodies. Consider the plasma rifle in dwango5 map1, to get it you have to activate the lift and wait, allowing other players to come ambush you as you attempt the ride up or to run up the steps to the backside and nail you as you grab the plasma. If you get the plasma, you have a choice of which way to go, back down over the lift, out into the yard or back down the stairs.
Mazes are not much fun for most deathmatchers. Some of the better designs allow players to catch sight of each other across fairly large areas of the map, although they may have to navigate through some structure to engage in battle.
Use of diffences in elevation is often welcome in a map. Look at gothicdm map02, the gray castle, for an example of "genius" in a fairly small map.