Also, I've got a few tips if you're interested:
1) ALWAYS ALWAYS ALWAYS put borders around your level. If there's no borders on certain sides, that is a bad sign because if that level is a CTF level and someone captures the flag and touches the outside of your level, all others will get flooded with "<player> captured the flag" messages until that player leaves or gets roasted.
2) Try and think up a particular and original concept for your level, that keeps it interesting for players.
3) Practice, practice, practice. As they say, practice makes perfect. If someone doesn't like your level, keep making levels because every time you make another level it will always be a little bit better than the ones you made before.
Trust me, I make levels ;P.
Those should help. Oh, and use a good, non-limiting tileset, because sometimes certain tilesets are VERY, and I should say VERY limiting.