Fair warning... spoilers abound.
Let me start off by saying that I also think La Luna was a phenomenal short, and very much enjoyed it.
I'm glad, in a way, to see that some other folks have lukewarm feelings about the movie. I saw it yesterday, and while it was enjoyable, and certainly a "bad" movie, I also found it very underwhelming. There was plenty that was done right, of course: it was fun and entertaining. And, like any Pixar flick, despite the characters being computer generated "cartoons", you don't really think of them as cartoons. Pixar films are known for their emotional immersion, and this was no exception.
But, three specific things stand out in my mind as disappointing--two story related, and one visual/technical.
First, the story lacked a clear protagonist/antagonist. There's a clash between mother and daughter, but there's more than enough reason to feel for either one or simply both of them; there wasn't much to say that one was right and the other was wrong, or one was good and the other evil. Once the mother is transformed, it's easy to sympathize with her, but we can't exactly call Merida the anti-hero as she's also the one who has to then save her mother. An interesting attempt to juxtapose plot roles a bit, but I just don't feel like it worked as nicely as may have been envisioned.
Second, while the story in its entirety was unique, on a smaller scale, all of the individual elements felt very tired. A mother and daughter butting heads is nothing new. The three boys reminded me of the mice from Cinderella. There was an ancient legend that (surprise) turned out to be absolutely true to the letter. (Consider any adventure/rpg video game.) A strange witch in the woods sells an expensive gift, but the user ends up paying a nasty price even after that. The mom gets turned into an animal, and they need to find a way to turn her back to a human. (Beauty and the Beast, Emperor's New Groove, to name just a few) And, at the last possible second, she is saved. (Tangled, Beauty and the Beast, among others) That probably sounds a bit cynical, and I realize that tropes are tropes because on some level they actually work... I just didn't feel like any of it was done with an unexpected or novel twist at all.
Third, I was disappointed by some of the visual work. I know it's a bit sacrilige to say that about a Pixar work, as their stuff is typically superb, and *most* of this was just that, EXCEPT the animation of the mom in bear form. I can't quite put my finger on the exact technical issue, but I think it may have been in the way she was animated. Despite all of the characters being "cartoons" on some level, their locomotion was consistent with actual human motion. The mom, however, wasn't moving in a way that looked human-like or bear-like, or even a cross between the two. It as though not just her visuals, but now also her animation was cartoonish. That's fine for Roger Rabbit where the character is specifically a cartoon, not so much when it breaks the sense of immersion. I hope I explained it right, anyway. It just, to me at least, looked amateur.
Anyway, that's my overall take away. Wasn't horrible, I'll probably end up buying the DVD/Bluray when it comes out, but it's nothing that has me too excited.
Have you seen the movie??? It IS a "girls movie" no matter how they want to spin it. The triplets are cute and good comic relief, but are in it very briefly. There ARE certain Pixar movies geared towards certain demographics. 
