The "What's so important" line sounds like it was looped- which is probably why it sounds funky. I think Luke's "but I was going to Toche station" line was also looped, and to me it was really his only cringeworthy delivery in the whole film (the audience actually laughed at that line when I saw the SE in '97). I think it's hard sometimes for the actors to loop the lines- keeping in sync with their mouth movements while at the same time trying to re-create the proper delivery of the lines.

I think overall Harrison did a great job in SW, an even better one in ESB, but seemed to lack enthusiasm a bit in Jedi.

I still think the performances of everyone involved far surpassed the performances in the PT, however.