I think that the real reason why so many Protestants hate the Rosary can be summed up by these reasons:
1) Bad will: This one is obvious, but it needs to be said. Many Protestants are bad willed and will think that Catholics worship Mary based on the Rosary alone. Even if you tell them we don't, they'll still say we're somehow worshiping Mary without knowing it. They don't want to accept anything about Catholicism because Catholicism is too difficult.
2) Stupidity: Some people are just inexplicably unintelligent and the very idea that prayer without Latria worship could be possible is beyond the scope of their very limited imaginations.
3) Anti-Catholic Upbringing: Many people are brought up to hate the Catholic Church and not even understand why. They hate crucifixes mainly because we use it. They hate the sign of the cross because it's Catholic, even though there's really nothing wrong with it. They hate the phrase "full of grace" because it provides a testimony to the immaculacy of the Blessed Mother. They hate the Apostles Creed because we say it. If they do say the Apostles Creed, they edit it in all kinds of ways to avoid making it sound like they agree with us and so on. They just hate anything and everything Catholic.
4) Impatience: Most people are impatient, and Protestants are no exception to this. The Rosary is something that takes time to do, and most do not want to take the time to go through it.
5) Misunderstandings: Many Protestants misunderstand various aspects of the Rosary and so they come to hate it because they're afraid of it. They don't understand why we pray to Mary, so they shun it (but they're so willing to accept Buddhist/Hindu prayer form, which I think is rather strange). This isn't the same as stupidity, which is self-imposed in 99% of cases pertaining to this matter. This is about being mistaken about the true nature of the Rosary. If more Protestants would take the time to understand the history behind the Rosary, they certainly would not shun it.