Nicely conceived and executed; I haven't finished it yet (EDIT: stuck on the first sproing level, may come back later) but the puzzles feel just the right level of crunchy. A few suggestions:
- Fullscreen option
- Level select as others have mentioned, maybe combined with a progress indicator
- Complex levels might be easier to 'read' if all blocks at a given level shared a colour (but I appreciate this might get too noisy once all the special blocks come into play)
- Highlight the object under the cursor; sometimes it's not the one you expect, especially at odd view angles
I like the understated sound a lot - feels very tasteful and atmospheric.