Nothing specific. It just must not be a leveling map. And TWI decides what is a leveling map and what isn't.
Just make sure enemies can get to you and you can't shoot them through a slit in the wall while they can't reach you. As long as players can't use the map to level up their perks without having to work for it you should be fine. Post in the topic in a similar form as the others and be sure to add a working download link so TWI can test it.
After a few days (or weeks in this case as they are on Christmas break) they will update the whitelist and post in the topic about it. If your map didn't qualify they'll tell you roughly what you have to change.
If the map is good or good looking doesn't matter. The whitelist isn't meant to only include good maps! It's just there so players can't level up their perks on maps where they just have to tape down the fire-button.
If you aren't sure if your map is white-list ready you can just post it in the beta section and let other people try it and give you their opinion.