Faaaaaan-tastic! Straight-up amazing.
Now, a few minor things to consider: the animation on their, ah, "rumps," to use the good Doctor's terminology, seemed a bit choppy when they were walking, so maybe clean that up a bit.
It's been mentioned before, but the voice tracks could use a little alteration - Doc sounded a tad static-y, and Derpy/Ditzy seemed like there might have been a bit of an... echo, I guess? Not like yodeling in the mountains echo, but kind of like speaking near a tin can. Although, that could possibly just be the way her voice sounds. It *is* pretty squeaky.
Next, tiny little details - I think I saw a missing outline when the Doc raised his far leg in a shrug-type gesture, or the black line around the Doc's eye not quite matching the white of his eye, etc., etc. Assuming time allows, really look it over with a super-critical eye, like you're a pony-hater just BEGGING to find any little mistake to criticize. You know, give that last extra layer of super-fine polish.
Finally, what I'd say was the biggest thing would be the camera angles - specifically, the lack of them. Straight-on side view, that was it. I'm sure it'll improve with practice, but don't be afraid to get active and dynamic! Just an example: when the Doc's looking around for the first time, switch to his P.O.V., swing side to side, and then - SUDDENLY! DERPY! - her face swings down to a frontal view right in the Doc's - and the viewer's - face! You know, stuff like that.
Again, you've got great stuff here - I'm just looking for anything that can help push it over the top to near-perfection.
Anyhoo, love it, and I know you've got more, so I hope to see more soon!