Just really fun. It's so stupidly simple, but done in a really nice way. The shapes are so simplistic and clear there's no guessing whats happening once you've seen the enemy once. Which leads to solid gameplay and thoughts behind your actions. Even the simple 2d over 3d with shadows just has a very appealing look.
It's simple and fun, exactly what it should be. If I had suggestions it would just be tweaks and general improvements, more enemies, etc.