Maybe make it so that the thread is locked (to all but the poster) if there isn't a download link and/or pictures? (example) Dr. Spud is working on a new map, but it's not quite complete, so all he has is a short description, name, and gamemode. If he feels like it, he could post a picture or two. If he has a download link, he can get off his ass and post a few screenshots. If it's not been compiled yet, he can post some screenshots. Once he has both, and people can download it, the thread gets unlocked.
The only thing you can comment on with screenshots (omitting special circumstances) is no cubemaps, lighting, and/or 'it looks nice.' With a download link, you can go into the map and actually give some feedback - which is what we want to do (we're not showing off as much as we're trying to make a good map). [BUT if you only have a download link, you could make a terrible map for shits and giggles and make people download it and such. With both, you can see what the map will be ahead of time, know what to expect, and have the ability to try it out and give feedback. It also prevents the 20 replies to "OMIGOD SECRET MAP" in an hour - a thread of flames.