Scripting. Not the plot-writing kind, mind you. It is essential to have some sort of scripting engine if you're not relying on procedurally generated content - and it would take too much to write a procedurally generated interesting exploration game.
The basic functionality is there (object activation, map-specific events), but a lot still needs to be done - connections to other maps, associated speech, and modification of map collision data.
I also need to make at least a few more frames for the protagonist, since as you can see by comparing screenshots so far - he really only has one pose, and he floats along the ground. I am a terrible animator though, but I'll have to try anyway.