Oh wow this thing is older than an Egyptian coffin. I frankly feel Rowling DID rip off a few things, but not exactly the entire plot. Also, I don't find Dobby and Gollum all that similar. But Old Man Willow and than strange Whomping Willow are too alike for my liking
Not to mention Gandalf and Dumbledore.
Then again, most contemporary fantasy authors have, either consciously or unconsciously, ripped something off LotR.