After the first few days of moderating the map and mod uploads, I'd like to give everyone some guidelines here to avoid rejections:
1) If you make a new version of your map/mod, please do not upload it as a new item. If you view the old one on the library, there should be an edit link on that page. Using that you can then update the existing entry. This also has the benefit of maintaining your download count and any comments other users have posted. As a side-note, please wait for the first one to be approved before uploading a new version!
Important exception: If your upload is rejected for some reason, please do upload the fixed version as a new item. The moderation system doesn't seem to have any easy way to see that an item that was *not* approved has been updated.
2) Please use an appropriate preview image. Ninety-nine percent of the time this will be an actual screenshot. For maps you can do this in the Galaxy Forge; for mods and replays it should be an in-game screenshot. Do not use random unrelated images you found on the internet. Preview images should ideally be at least 640x480 pixels in size so they can give a clear representation. If you've made a mod that is not easily represented by a screenshot, then at least make a simple logo or something.
3) Please make sure to give your files unique names if appropriate. For maps, you might try including your alias or initials in the .galaxy file's filename. Otherwise, players may end up with several different conflicting "Solar System" maps by different creators in their galaxy folders, for example.
4) This one should be obvious but please make sure it actually works before uploading it. We've had a couple of maps uploaded that crash the game on start, and some that instantly result in victory or defeat due to incorrectly configured teams or homeworlds.