The way I see it, most of the Nero dislike harbors from the fact he takes the games' spotlight away from Dante, which I actually understand and is rather justifiable. But as a character, I find him likable, personality-wise at least, and ignoring the fact we know next to nothing about his story.
Nero just seemed to me like a better version of DMC3 young Dante: he was cocky, sylish and trash-talk, but had none of that arrogant, indiferent and prickish attitude that Dante had towards everyone and everything. He actually seemed to have a sense of responsibility, actually cared for other people from the start and, most importantly, he had a struggle, something DMC3 Dante lacked for the most part. Every good hero, video game or not, has to have a struggle to be interesting, IMO, even if it's something trivial. Dante fans were probably too fond of the idea of a hero not giving a crap about anything and just doing things for fun than when they made the contrast, Nero looked too angsty. But I think he fulfilled his role as central protagonist much better than Dante ever has, and surprisingly, Dante fulfilled his new role as deuteragonist perfectly. I can't honestly see how DMC4 would have worked if it had been all about Dante going to fight the Order of the Sword just because, while Nero is the one with a story full of conflict (just think, DMC2 all over again, where Lucia was the one with the conflict, while Dante just happened to be there).