I posted some of my thoughts on GitHub before, but thought that it will be interesting for others as well:
The editor partially simulates the scene, by playing sprite animations, performing background movement. Toggleable/configurable.
Everything is in WebGL (in PIXI), and should resemble the results seen in ct.js game as close as possible.
With the addition of hand-placed sounds and particle systems (SOON), the editor will allow for a more immersive workflow, that will greatly contribute to the level design pipeline (more interactive WYSIWYG in editor > less testing in a debugger. It is also just cool).
Copies and tiles should be transformed as layers in Photoshop or other graphics editing package — with multiple selections, rotation and scale handles, ctrl+C, ctrl+V; bring artistic value and composition to the workflow.
Undo/redo must also be present, as it is easy to mess the things up when moving lots of things around, especially when using no grid (#76).
Do provide tools for extended scripting with custom copies' parameters (#40) and triggers. This will also pave an avenue for level designers that don't code.
I also established a foundation for the editor, more pecisely the renderer and navigation. There are still tons of stuff to do, though.
Do you have other cool ideas for the room editor? Post them here or at GitHub!