I'll admit to being a pretty big sucker for shoujo, but what really endeared Tonari no Kaibutsu-kun to me was Shizuku's startled reaction, the one where her pigtails stood straight up in the air, as well as Haru's hilariously random behavior. Even better, I think this show is an example of Shoujo Done Right, and is free from a couple obnoxious tropes I will mention later.
Story: Lighter on plot and heavier on character development, although it wasn't the huge or meaningful type of development; mostly just looking at the difference between Shizuku in episodes 1 and 12. I thought event sequences/timing were a bit muddled in their focus, probably a result of widening the number of characters the show focused on, and of course we were left with a bunch of loose ends at the end of episode 12. Obviously the manga goes on, and I actually might just look this one up to read. Viewers may be intrigued that Shizuku and Haru sort of play a tennis match with confessing to each other, without much of a conclusion on their future or if/when/how they get together (although you could rightfully argue that they essentially are together by the end of the show).
Animation: Delightful. There were some really good instances of depth and lighting, even on the last episode.
Sound: Pretty good, but I didn't like all the voice actors for the side characters.
Characters: Lovely and each with their unique quirks, which all served that comedic-meeting-of-personalities in a way that reminded me a lot of Durarara or Kaichou wa Maid-sama. Not all of the characters were explored fully, but I thought it was a well-rounded group. The development was there, and I'm glad it erred on the side of weaker development instead of on the side of exaggerated development as a result of melodrama (which the show was lean on in general anyway).
Shizuku in particular is a refreshing female lead who is still figuring out her feelings at the end of the show, and seems happy to plug along at her own pace rather than rushing to a conclusive confession. The show is also happily free of the "bitchy female love rival" trope, as well as free of the token kidnapping of the female lead by an enemy of the male lead.
All in all, this is an entertaining little series. The overall tone is comedic, but it doesn't shy away from breaking into some more serious and romantic moments, which makes the show more touching than sappy. If you already dislike shoujo, I don't think this show will change your mind about it. If you like the genre but are sometimes frustrated with its predictability and tendency for melodrama, then you are in for a treat on this one.