Besides blowing all the reveals, seeing the PT first prepares the viewer for an entirely different type of movie than SW was ever intended to be. I'd be disappointed in ANH too if I was expecting a sequel to ROTS.

How well would the Berlin Chamber Ensemble go over as a closing act to a Mellalica concert? Regardless of your preference, your mind is just too scrambled to seamlessly head in that direction.

Regarding the reveals, there are a LOT of them. For example, when Luke says "what a piece of junk!", compared to the ships in the PT it really IS a piece of junk. So that line is ruined. We've already met Jabba so that reveal is blown (well, the SE blew it already I guess). Of course the Vader reveal, the sister reveal, the Yoda reveal, the force reveal (Obiwan in Mos Eisley)....