I love a good flowing layout. Gameplay is very important too. Implemented difficulty levels, proper ammo, health, armor, and powerups, and also enemy distribution. I personally hate teleporter traps (enemies just spawning in to surround you), and also no one wants to have their reward for finding the red key to have to face down 4 cyberdemons with just a shotgun. You also don't want to be able to be killing every single zombieman with the BFG and never run out of ammo, so balance is key.
I love detail, but to me the best detail is using textures properly. There is a such thing as too much detail also. To me the key is when the player has finished the map there should be a sense of satisfaction, like they've accomplished something. My favorite wads are also very clean and well-put-together wads with new music, proper mapinfo, and having an overall theme. However I do enjoy community wads as well.