I didn't like Dirian telling Edmund he's not in charge. I think Ed and Caspian though of each other as equals (except at Goldwater, but that's the whole point there.) It would have been better to not have the scene in there.

The High King and Queen drove me a little simply because it looks like the script writers didn't know their source material. Its an error that should not have been there. Period. Exclamation mark!*

*to quote Reep

okay the way I took the whole High King thing was that I think it was Caspian's way of telling the crew that the Pevensie's were kings and queens before he was king.....

Well, I didn't like the high king and queen of narnia bit, but I understood that the filmmakers thought this would be a quick way to tell the audience that didn't read the books that Ed and Lucy have more power than Caspian. But then in the rest of the film, they said the exact opposite, which made the first scene completely pointless.

I didn't particularly like the scene where Lord Rhoop is "Rescued"
It goes something like this:

Our heroes are sailing through the G. M. (Green Mist). Suddenly, they hear horrible cries. They spot a man on a tiny island who looks like your generic old, dangerously crazed gentleman. "Oh, an aged crazy dude. We'd better not get involved." But wait!
"LOOK! That guy's got The SWORD! "
"You're right, Ed! Quick, men, grab him! I know we'll probably give him a heart attack, but we must get that sword!"

But when Dirian told Edmund that Caspian was the King and the one in charge, that was when Caspian should have stepped in a defended him and he didn't.

Well, Caspian tells Drinian to do what Edmund has just suggested, doesn't he?

I think it adds to the plot, I think that dirian was in line with telling Ed that, being that it was Caspian's ship. Ed, in the movies, has to deal with being a king under a ruling king, first with Peter and then with Caspian as he pointed out. Yet I think the ending where Caspian says, "I think of you as my brother Ed" restores some of Ed's adverse feelings, to understanding. (They both hugged and got over it in the end )