The main problem was coming up and implementing a metric for "correctness" was too annoying. I could do squared error matching the player's curve-through-time against a stored one, but I find the game of "move along this curve at constant speed" kind of irritating. Doing anything like distance-between-point-and-bezier-curve would involve doing some actual math, and I don't really care to work out the formulae or to implement it in actionscript.
So I did something much simpler, based loosely on a description of a contour drawing exercise I found in Kaupelis. No quantitative grading, but I found it very easy to qualitatively judge myself line by line; the idea is you're basically tracing, but the image and the cursor both vanish as soon as you are actually drawing. Thus for the duration of each curve you have to memorize a little bit of the contour of the image, and play it back through your hand. I was impressed how bad I was at doing this except for very, very short strokes at first, but I'm starting to get a little better:
(photo of old cigar dude gotten from here)
The game saves everything you draw in the clipboard as postscript, so just paste into a text file and save it with a .ps extention and view it with your favorite postscript viewer. I'd like to make it easy to load your own images from a URL or whatever but as far as I understand the flash security model makes that somewhat tricky...