The book has been a political hot iron since it was written. There is nothing in it that should be so controversial. It's a good story which espouses individualism and self reliance. I think statists hated the idea of judging yourself by your own standards of excellence rather than worshipping standards of the collective. The fact that Hal loved it makes me respect him even more, if that's possible.
Don't discount misogyny either. Ayn Rand was an intellectual powerhouse who terrified male socialists.