Could have been worth playing. Reasons it isn't now (please take this as being constructive):
1. Not maneuverable: The invisible box around Rainbow Dash and/or the clouds is too far outside the visible boundary. There are a few too many clouds given the number of given cloud speeds (either less clouds, or make the clouds travel more similarly in speed). Also, the "rubber band" reaction of RD to the mouse should be quicker (the TV-show RD would put this one to shame ;-) ).
2. Looks: RD has no animation and the "Sonic Rainboom" looks like nothing more than a picture blur. Also, animating the boom (start and continuing effect) and allowing the RD to punch through clouds while rainbooming would be better.
3. Quick loss of interest: There only seem to be 3 types of items in the game (R.D., clouds, elements). Adding more features (and animating them) would add quality.
1 star for each of the above, but #1 was the most annoying. :-P