Just like Joyce's Ulysses, Dostoevsky's work, etc. those all are books that some people consider hugely important, yet because of their difficulty, they won't touch lives of many. Even students of literature often don't understand these works. And so, I am sceptical when they get pronounced as extremely important. 'Important to whom?' seems to be the crucial question...
Paradise Lost is one of the single most influential and brilliant pieces of literature in the history of the English language.