You may not play the single player missions, but if you want yours to be anything like the plotline you're gonna have to. Something that can get very boring at times is making the terrain. Don't just put a little bit of this, a little bit of that here and there either. You don't have to use every type of terrain, though you should use at least 5. Make what makes sense.
Doodads are our friends.
So use a lot of them. Don't overdo it either. Check through your mission. Don't make the characters OOC, whatever you do.