Focus on drawing a layout for the map.
Gameplay is
everything.
Be sure there are two entrances to every subsection
and one large gathering (outdoors) place where most players would spend for the first three waves.
A balanced map where humans and zombies have decent advantages (and where humans do not get shittons of kills) is an enjoyable.
As for details, the least you could do is align your textures and place them so they do not look repetitive but make sure they do not appear stretched or TOO large.
If a texture is repeating in your map and there is no other way you could make it less repetitive, is to make your map somewhat dark.
Do not forget about decals, overlays, prop_static, and fun_detail as they provide all the small details in your map.
As for the theme of your map, no one cares but be sure it is a strong theme you could work with and people would enjoy.
Do NOT base your theme on some other game or map because it will either look crappier or you will abandon it.
On top of that, present your map with screenshots (DON'T POST FUCKING LINKS) and with a paragraph with features, info, file size, etc.
A map that took less than 4 hours to make would be terrible.
There are thousands of tutorials and advice on the internet for mapping.
Be aware of your environment in the hammer editor.
Learn every single entity so when you do make a map, you will know where to start.
Mapping is not meant for everyone who can try.
(This goes for everyone new to mapping)
And take a look at you map before releasing and ask yourself,
"Does this look like shit?"
