Ultimately we read to be entertained. So when a character shows signs of being the kind of person we'd be bored by in real life (usually because they're wet, annoying, bland, full of hot air or 2-dimensional, rather than because they're good/bad) then I tend to think that's when the 'not likeable' complaints will kick in.
So Ellen - I think you've landed on it with the word 'fascinating'... It's not about nice/nasty. Because if a character is awful but amazingly so, then they can become fascinating for that.
How else to explain the hugely successful Dexter novels by Jeff Lindsay - not to mention the Golden Globe-winning TV series? Because IMHO Dexter makes Heathcliff seem like a sweetheart...