Akward and repetative
The graphics were simple, but they fit. It was clever to notice that while the base 4 had unique colors, the other numbers were all mixes of those four bases.
However, someone pointed out that it is "awkward" for laptops. I don't think it's just laptops, but the mouse would probably have helped. The buttons and the way I had to move with the mouse and that (obviously) the game became unresponsive once I moved my mouse off screen accidentally was essentially bothersome.
Another point is that the game was repetative without any change except for difficulty, which was barely noticeable until about level 6 or 7 when the multiples began coming. Maybe mix it up with some colors and different music?
Maybe even a level tiering, like making different worlds, levels, and achievements, similar to something like Star Fox's idea of having about 2-3 levels per world and then make a few worlds.
I mean, you load from offsite, and the game is under a meg, I'm sure it's feasible.