I didn't get around to this issue in my review of Episode III, but I was puzzled by Obi-Wan's victory over Vader. Although the movies indicated that Obi-Wan was one tough Jedi (e.g. he singlehandedly took out Darth Maul after the latter had beaten Qui-Gon, and Obi-Wan jumped into the fray all by himself against that robot guy [his name escapes me] with 4 lightsabers plus all his droid cronies), clearly we learn that Anakin is better.
Of course, we knew going into the film that the two would fight each other, and we knew that Obi-Wan couldn't die, and that (probably) Vader was going to be seriously injured. (I say probably because Vader could've been injured in a different way.)
What I thought was going to happen is that Padme would've intervened--"Anakin, no!"--and this distraction would allow Obi-Wan to get away. But no, what actually happened is that Obi-Wan flat out beat him.
How so? Well, Obi-Wan says, "Don't try it, I have the high ground" or something like that. At the time this literally referred to his physical location on the bank. But perhaps in a deeper sense, Obi-Wan wins because he is right and Anakin ultimately knows that he (Anakin) was in the wrong.
Finally, why doesn't Obi-Wan finish off Vader? Three possible reasons:
(1) He can't bring himself to kill his old friend.
(2) He thought Vader was dead for sure, anyway.
(3) He still believes Anakin is the Chosen One. (And this is fulfilled in Episode VI, remember.)