I have almost finished my level editor. It currently saves the world data to the models folder using jME’s BinaryExporter (wow, that saves a hell of a lot of time), and you use the mouse to place tiles, and the scroll wheel to cycle between tiles and objects. Right clicking removes the top-most tile (basically, the first one the picking ray hits).
I’m going to try using the lighting system to add some moody effects next, after I create some more graphics. It turns out, making nice tile graphics really isn’t that hard after all.