I thoroughly enjoyed this episode. The little things were paid attention to, and really boosted the overall feel of everything. I liked Spearhead. No one seemed to be out of character, which was great. Pinkie in particular seemed a lot like the Pinkie I remember from the earlier seasons, which I loved.
This episode made me a little softer toward Flurry Heart as a character, although I still don't like her design (her proportions bother me the most). I liked her mannerisms, they were pretty cute. As another user already commented, her nature still seems quite a mystery. Some things that remain a question about her character for me are:
1. If she can teleport wherever she wants to, I figure this should lead to some major misadventures for her parents? I could picture her easily getting distracted and ending up... well, anywhere. How do you discipline a child that can teleport away from you at will, and literally barrier themselves from the world if they'd like?
2. It's official. An infant can overpower Twilight. Coupled with her intelligence, what does this mean for her in adulthood? Will she mature faster than a normal baby? Just imagine if she turns to the dark side... she'd be a very scary entity.
3. Flurry Heart can fly, despite being younger than the Cakes. Her eyes also have never appeared "normal" the way a foal of her age should've.
Flurry Heart has been given a lot of attributes that make it feel almost like she could take the spotlight herself. We'll see... although I personally hope this isn't the case. Her struggles with magic will be interesting nontheless.
Again, great episode, really hope the rest of the season keeps up the good quality!