I got to this discussion late, and of course King Darkness' .gif speaks for itself. Anytime anyone says that the PT fights were awesome, the sequence in the .gif is the first thing that comes to mind. The two sequences in Episode One were the best of the Prequels. The mirrored shot from RotJ in the beginning of RotS was really nice, but otherwise, they're all poor. I don't know how four arms with four lightsabers lose to two arms with two lightsabers.
Anyways, the reason for my post is that I actually think that the Obi-Wan/Darth Vader fight in ANH is the most important fight and contains the most important moment from the entire series (Prequel and OT). When Obi-Wan and Darth Vader are fighting, that's when the rest of the crew all make it to the hangar where the Falcon is waiting and ready to launch. As Luke is running across the way, he sees Obi-Wan, and Obi-Wan sees him. It is at this point that Obi-Wan ceases fighting and apparently starts his One-with-the Force ritual, and is cut down by Vader.
I believe that Obi-Wan felt that he had accomplished his 'mission' by essentially introducing son to father, or redeemer to sinner. I don't really think that Lucas had intended it to come across that way, but the look that comes across Obi-Wan's face is pretty powerful. Added to the fact that after giving that look, he STOPPED FIGHTING.
As others have said, the lightsaber battles in the Original Trilogy all had emotion and gravity, and a greater purpose than just a 'wow' factor.