I am thankful that the first time I saw The Princess Bride I did not know what it was. I was still young, and I'd never heard of this movie before. It made everything all the more fresh and surprising.
The very heart and soul of the movie is a traditional fantasy where true love conquers all - even death - but performed entirely with half a smile. It is very well-written and funny, and rarely are comedies played so subtly.
One of the reasons I say I'm glad I saw it without knowing what it was is because the movie is so quotable. Unfortunately movies like this are sometimes ruined by our culture by having constant references take away from some of the humor.
So I'll just say if you've seen it, you know it's great. If you haven't, you should give it a watch so you get all those references to it you keep hearing every day, but also try to forget them. Let it come to you like it's still new.