I started a sith clone at one point, but lost interest because I knew I would have to replace JED, and knew I couldn't come close to matching it. JED has one feature that puts it above all other editors: The grid. The very idea of a grid that can be rotated and scaled and aligned to anything isn't present in any other editor anywhere. But it has one huge limiting factor too. It needs JK. It requires a CD (or a no-cd hack), it only works with JK resources; etc. Now it might be possible; it will be interesting to see what Jon'C produces. I'm just afraid hes falling into the trap of cloning JK exactly. If I could get a more general purpose editor, I would continue with my portal engine. But it wouldn't be a JK clone.
If JED supported more image file formats than just mat (specifically TGA would be nice..) and didn't require JK, I could use it as-is, though I'd have to integrate scripts the hard way.
Really, I think the lack of a nice sector-based editor is the single biggest thing standing in my way. It's not something I want to write myself, mostly because I would always be comparing it to JED and always be coming up short.
Also it would be nice if JED's graphics support was updated a bit. I distinctly remember there being certain combinations of screen resolutions and video cards where it just didn't run, or the 3d preview didn't work, or something. And it's probably about time the textures in the 3d preview were filtered.