These seem to come up more often than anything else, so I thought I'd preempt having to respond with almost duplicate answers if these questions come up. So these are the currently planned features as they are completed or being worked on or designed.
What will be editable or exposed via scripting?
-Everything. (including file loading, opengl shaders, particles, drawing using raw vertex arrays, etc. *note* this does not mean everything will be done by the time 1.0 is released however.)
-There will be no support for palettes planned in 1.0. (Sorry but it's not the 80's anymore. Instead this is replaced by very powerful and flexible OpenGL rendering capabilities and 32-bit image support.)
Only the most basic stuff for now. (Depends on what editor functionality exists I suppose) These include:
-Maps with a maximum of 16 layers. (Technically there is no reason to even clamp this at all except for memory concerns. If you use more than 3 layers though, you are probably doing something wrong.) Any layer can also have opacity and tinting.
-Spritesheets and textures. There is no "image editor" built in. Use photoshop or graphicsgale to edit images!
-Tilesets. Tilesets can be any arbitrary size, however they must use the same texture (non negotiable). Thankfully image sizes of 2048x2048 or more can be suggested, though if you use higher than this some people will not be able to run your quest (game).
-Tiles. Tiles can be any size, *cough* 16, 32, 64 *cough* though every tile must be equal for the entire game. You cannot have one tile 16x16 and another one 128x128. Use sprites instead.
-Animated Tiles: This was almost painful to work out non-animated and animated tiles living side-by-side, therefore all tiles can be animated! The reason I can get away with this and others like rpg or game maker cannot, is because I have over-optimized these on a very low level. :) These are very fast. (I use the term 'fast' here, but my "fast" is not equal to others' "fast". So there should never be any performance issues even on low-end hardware.)
Also they are fairly small, only about 64 bytes in memory. This means that a tileset containing 65535 tiles only uses 4 MB of memory, with 4 + (4 * num_frames) Bytes for animations.
-Sprites. Support for blending, rotations, scaling, animations, etc...
so far each have quite a bit of space reserved which can be edited and loaded when the game is run.
Everything else: (Under construction) *no code (or interfaces) exists for these yet. I'm working on it as fast as I can!...
Currently working on: Low-mid level and scripting design, along with data saving/loading.
Todo: Menus and Script entities. Battle engine. Characters. Map areas. Rendering.