The rain drops are too spherical as they traverse the screen. The friction plus surface tension of a water on glass pulls the tail out significantly farther then represented and also varies on whether the drop is following a path that's already been traversed and therefore has less friction. It would be really really cool to see that effect added in.
Each 'drop' of rain is really a circle painted using the reflection scaled to the size of the drop. This approach is very fast, but won't scale well with odd-shaped drops and larger drops. With a small enough spherical drop, it's a good enough approximation for refraction.
With larger drops, you'd start to notice that the reflection isn't a refraction (i.e. the reflection is cutout in the shape of a circle, rather than truly refracted as in a drop of water).
Other approaches could be adding more expensive texture transforms, or WebGL + a refraction shader for bigger and/or non-spherical drops.
Overall a wonderful demo.