Let me begin, then. To clarify, the whole "adult man ends up in a romantic relationship with a girl he's known since she was a child" won't even be addressed; I maintain that it can be done well, and is therefore not a sin. (Case in point: Dangan Honey.) No, there are much more important things to discuss. Such as:
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[*] Firstly, there's the fact that that many people (myself included) were shipping Rin with Kouki and his mum with Daikichi. But who cares, that's easily forgivable.
[*] Then there is the time-skip itself. I remember reading and thinking "Wait, did I miss a few dozen chapters or something?! What happened?!". There is no introduction to it, nothing to let the reader know he hasn't missed something. Still, this too can be forgiven.
(If there is one faint parallel one could draw with Dangan Honey it's this one, because the time-skip was abrupt and had no fore-shadowing from previous chapters. But here it was at the very least passable, because its introduction let us know what was going on.)
[*] There's the fact that Daikichi was acting as Rin's parent and guardian for all those years. This ups the creepiness quite a bit. But we're still within the forgivable zone as far as I'm concerned; at its fringes, but still within it.
[*] There's the fact that the narrative had established several things from the beginning, only to awkwardly throw them out the window when it became suitable. No prior fore-shadowing, no doubts expressed at the beginning, nothing. Just "Everything you know is wrong". This is very hard to forgive, but a manga that was otherwise amazing could have saved it. Maybe.
[*] And lastly, there's the absolute most unforgivable thing about Usagi Drop: It retro-actively turned the entire pre-timeskip arc non-wholesome. See, the original premise was "This girl is family, and someone should take care of her. Daikichi manned up when no-one else would." Post-timeskip, the premise magically becomes "Guy gets suckered into caring for his grand-dad's mistress's bastard and remains single into his forties as a result." This cannot be forgiven.
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