For me, watching Usagi Drop was an exceptionally needed experience with healing, because when I watched it I had just finished reading Muv Luv Alternative, which is a very… “traumatic” visual novel. For this reason I don’t actually remember much of Usagi Drop when it comes to the story, I just mostly remember it as something that helped me relax.
And it really was good at doing that, but it was also the first show I saw that had an almost believable parent-child relationship, which was an extremely pleasant surprise. Lately there have been shows that have handled this theme even more believably (Barakamon, Amaama to Inazuma, etc.), but Usagi Drop was my first experience with this particular theme and it was by no means a bad one.
There is, of course, the matter of how the manga continues the story, which is the main reason I’ve never returned to Usagi Drop – neither the manga nor the anime – since watching it that first time, and I probably never will (aside from possibly rewatching parts of the anime). The manga ending is not something that’s easy to forget or ignore, after all.