One of the reasons I like this is because I feel that Granny Smith is a very underused and unrecognised character who deserves more attention, I've loved her ever since the Family Appreciation Day episode.
I've never seen anything even similar to this before so the idea is incredibly original. Rainbow's facial expression is small and subtle, but you can still see emotion in there, I can totally see her saying 'WTF?'. Another thing I adore about this and a lot of your other stuff is that you've obviously put effort in, but the style is very simple.
Overall, really good, and original idea with a great art style and expressions.