Specifically, flutterbat. Fluttershy when she isn't a monster isn't very interesting IMO, but once she grows teensy tiny fangs and bat wings and all... the mirror scene in particular. Squee!
Other good things:
Rarity wearing a space suit to capture fruit bats, and ending up with a face-hugger.
"Fruit bat round up, fruit bat round up!"
Circling and chanting dour, grim ponies... and circling and chanting happy bouncy Pinkie Pie.
Spike taking credit for doing nothing whatsoever.
The main problem I had with the episode was that Fluttershy's argument was really, really weak. I guess she was outnumbered five to one, though. During a musical number even!
EQD posted a fic prompt to add additional magical mishaps to the end of the episode, so I wrote:
Overall I liked it. It was kind of fluffy but the adorableness cannot be denied!