Had a quick go at this. Will try and have a better play over the weekend/next week, but some quick comments.
1. I found it quite difficult to find my colour palette at first, tucked away on a 2nd tab. Maybe make that the prominant tab over Scratch Pad
2. If you rightclick in the sprite/animation/frame tree on the right, the menu pops up with no background, and there's no selection indicator. Looks a bit weird.
3. Related to 2. When you rightclick on "0 degrees", and click "Add New Frame", this only works if you right click on the text. If you rightclick on the icon, it does nothing.
4. The black "borders" are underneath when you enable the tiling previews, which is pretty good. I'd like an option to draw the borders with an alpha above too (only something like 64), so that you can still see where your borders are, but not interfere too much with the tiling effect
But yeah, looks good so far