I think a successful level editor requires a very strong graphically designed GUI. You need to have a solid layout. Establish a good hierarchy with different tools and what they do. Keep it simple, make it very functional and to the point. Try to develop a simple GUI that anyone could use, a 10 year old or your grand-mother for example.
It doesn't matter how many cool features you have, If people can't intuitively and functionally use it, due to poor layout, it won't be as successful as it could be.