I always found it interesting that, if you watch Star Wars in released order (original trilogy first, then prequel trilogy), it ends with a sad ending (The Revenge of the Sith), the happy ending (The Return of the Jedi) being lost somewhere in the middle of the story. But it turns out that the story is in fact much sadder if you watch in chronological order, even though this allows you, officially, to end on the happy ending.
It always seemed like it would be so wrong to watch the prequel trilogy first; I never had done it before. Granted, while growing up we only owned the original trilogy and Episode I. So we would either watch the trilogy or the single episode. When I got to college, I watched Episodes II and III for the first time since they first came out; I would watch the two of them together. Once I ended up with the whole I-VI set, I watched it in released order a couple of times. Then I decided it was time to try something new, to just try it and see what it was like.
When you are watching the prequel trilogy after the original, you've just come from seeing the defeat of the Empire. It all feels more like seasons of peace and seasons of chaos. So as you see the start of the Emperor's reign, you're thinking, oh, so this is how it all started. But when the prequel trilogy is the first part that you watch, you don't have the same sense of hope and history: you're watching the reign of terror begin for the first time and its end seems very far away. Everything feels much more tragic.
Then once you move on to A New Hope, the perspective is still different. Instead of watching everything fresh from Luke's eyes as he learns about the rebellion and everything else, you are watching with the idea of a broken family in your mind. He is the son who had his family taken from him and is now left to try and put things right. Yes, Luke helps Darth Vader change back to Anakin Skywalker in the end--but all the years they could have had together are lost. The family unit was ripped apart as soon as it began and can never go backwards. The Empire is defeated--but its reign was still at the expense of this family. And the personal can be very tragic.