I think part of the problem is that you wrote was was "wrong" was it rather than "what was different". But using "wrong" you've already put a negative tone on your essay.

Anyway, as far as purist go, I can't say I've had much good experiences with them. All I remember from when I was in LOTR fandom was them using every opportunity to tell everybody how awful the movies and PJ were and insulting the intelligence of anybody who didn't like the books. I peeked into The Hobbit fandom shortly before the movie came out, saw much of the same thing and made a hasty retreat.