Here is what I noticed.
In my opinion, the biggest plotholes are:
- Where is Sunset Shimmer's human version in this world? (Already brought up, but still)
- Is the entire human world a mirror of Equestria, or just the school? I mention this because why would Celestia and Luna be the principal and vice-principal of just a school of it were an actual mirror, since in Equestria they rule everything?
- Where is Twilight Sparkle's human version in this world? I know Pinkie Pie asked Twilight if she had a twin in another city with a dog named Spike, but if this place was truly a mirror of Equestria, why wouldn't human Twilight be friends with the other human main cast? Also, if it's just the school that is mirrored, why would Twilight live elsewhere? (This also goes with Sunset Shimmer, assuming her human version is living somewhere else. Unless Sunset Shimmer "got rid" of her, which could be believable.)
I can swallow the learning to walk and such very quickly for movie time reasons, but I can't get my head around the stuff I've mentioned. (For example, when Twilight just comes out of the portal, she stands up and walks over to the horse statue in a bipedal posture, but she later has trouble knowing how to walk, but this is purely animation error / time constraint / overlook.)