This is my honest opinion: Usagi Drop started off marvelously but then lost its luster in the second half and never managed to get it back.
I won't get into the big reveal because that was the least of my complaints about this series. It wasn't really even one of my complaints to begin with (considering how much I liked Hana to Akuma when I was a kid). Also, it's pointless to comment on other people's level of comfort with the way this series ended. Disliking it for whatever your reasons are does not necessarily make you narrow-minded nor do your reasons for liking it make you strange or crazy. Big reveal: opinions vary.
Anyway, my main gripe is as a someone commented earlier: the decline of the overall presentation. While I agree with cecropiamoth that the author did implement transgressive into this work, I also feel that using it as a plot device greatly contributed to the series' decrease in quality. The things that made the story what it was and made it so presentable suffered greatly for it in the second half. What do I mean by this? The series' steep decline from josei to shoujo and all that it encompasses for one thing (not a shoujo fan at all anymore); all the Kouki/Rin angst in the second half was annoying and gave off this sense of generic shallowness that I was SO not used getting from Usagi Drop. As another reviewer said, it unfortunately kind of went the soap opera route and made it almost painful to finish. The second blunder was the story's change in focus. Everything about the second half is used as a launching pad for Rin's "situation." Whereas the first half of the series builds upon character development, interactions and dynamics, the second half just throws all of that build up out of the dang window in order to make way for a cop-out time skip, some silly teenage angst and a poorly packaged epiphany. Things went from Daichi and Rin + their supporting cast to Rin, Rin, Rin, Rin and more Rin--that's it. The shift in story telling perspective was so major that it was literally like reading another series, or maybe like reading the same series in some sort of altaverse. My third gripe is that the time skip just left things all disjointed and lead to quite a few lazy explanations and gloss-overs. I have many other gripes, but those three are the most volatile, so...
The second half is very disappointing and just comes off as the mangaka having tried too hard to justify the Rin's resolve--which really didn't even need justification in the first place, so wtf?--and, in my OPINION, ended up falling flat when it was all said and done. It was like the artist just didn't even give a hoot about quality any more. If the transition from the first half to the second half didn't seem so much to me like some other artist came up with the storyboards then Usagi Drop would have been an absolute 10 for me.
In the end, Usagi Drop's second half serves as nothing more to me than a poignant reminder of loss of the earnestness and solidarity of the preceding half of this series that made it effing stellar. It has nothing to do with narrow-mindedness or Puritan feelings, I can guarantee it. The second half of this series was just not very well-executed. At all.
I read all 10 volumes and the doujin solely for the sake of completing the series, but I won't be reading it again, and I won't be purchasing the U.S. release--ever.