fun, good looking (not neccessary but shows effort), and balanced fairly is 3 of the things I look into a lot...
Clean, smooth-flowing, themed, good name, spawn areas, and good weapon placement. Asymmetrical maps are challenging; therefore, if you make one, I'll be sure to download it.
I likz t3h m4ps w!th Sn!P3Rs anD FuzOn Co!L5!!!!!11 No but really, I like maps that everyone else likes. In other words, I tend to look for somewhat popular ones. Other than that, a solid screenshot can get me interested. An interesting theme/idea also can help.
I like interesting themes, imagination, balance and detailed/good looking ones. All of these make the perfect map for me.
I look for an interesting theme that hasn't been done before. Appearance is nice, but that won't deter me. I like to see a good Infection map that doesn't favor the humans or the zombies, but creates an even playing field for both.
I troll the Competitive maps thread, and if I see well-put together structures, nice looking maps WITH PICTURES I will generally DL and check it out.