Hiya.
I don't mind WED, but it has some serious problems from an artists POV.
(1) "Hiding" a group hides it COMPLETELY. QuArK 'hides' groups by changing them into faint wire frames that fade over distance. Makes it easy to work on your current section without getting confused about just where that other hallway is, or how big the archway into the adjoining room is.
(2) No 'real-time' lighting, particles, animated sprites or models, etc. For a modern world editor, this is almost a must have, IMHO. (by real time, I mean moving a light and seeing it update automatically on the screen as you move it around the scene, as well as animated lights...like flickering, strobing or color changing, for example).
(3) Sounds can't be easily placed with a visual representation of their size/intensity/falloff. While not absolutely needed, it sure makes it easier!
(4) Easily adjustable texture stuff. Adding textures is easy...but adding a bump and spec map to a block isn't easy. And forget normal/parallax maps. The rigamaroll you have to go through to get a simple crate with a color, spec and normal map on it working is a royal pain. The editor should allow the user to select a mode/block/face and then add texture, bump map, spec, alpha, parallax/normal, luminosity, etc. right there, in the editor, by selecting one of those nice and handy " [...] " boxes that opens op a standard windows file dialog box. When the artists finds the texture he wants, it should automatically be added to the project file or .wad (in the appropriate folder of course). And all this should be in real time.
...there are other things, but I'm sure they've been expounded upon dozens of times already. For a great editor, I'd take a look at Unity and QuArK. Unity for 'everything', but QuArK for customization and 'building-style' stuff.