A good map editor knows what he can and can't do. Meaning he knows what skills he has, from inserting simple units to very complicated scripts. Your mission will depened on what your knowledge is.
As mentioned before, the story or the theme of the mission is a huge part. What fun is it to have a story of;
"You'll start here, go and kill this guy and return to base"
It's dull as you can see. Obviously. The more knowledge you know the more boundries you can press in OFP and "A LOT" more you can do. So to make a good mission my point is improve yourself constently in mission editing.
Also it has been said, ensure if it's to the meaning or point of your mission, have some place or something the player can heal by, there's no sense in having to crawl 1 mile to your extraction when your hurt.
Use your imagination to create missions you feel are in your array of interests. Do you like night operations, well you can use a wide variety of Speicial forces missions to behind enemy lines. If you like battles then you can create your biggest scenario you want.
A good map has a good story and a good plot, it's much like making a movie, keep the player on edge, i love missions where i can't always predict what's going to happen. Make it hard, but not too hard but intersting to keep the player wanting to play more.
Keep the suspence going just like your favorite movie. Also very simple missions are intersting to, as long as they work, like the ones BIS made in their official Single Player missions. I hope this helps, good luck.