I've never understood this at all. Mega Man 10 is my favorite Mega Man game ever, and I've been a fan of the series since the 80's. It has inspired level design with real care put into it, brilliant music written by pretty much everyone who's ever written Mega Man music, and really well designed, challenging boss fights (but different modes if they're too hard). There is nothing about it I don't absolutely love. The only way I can make sense of the hate for it is that people are blinded by nostalgia for the old games and can't fall in love with a new one.