I don't think that it has anything to do with Zack Braff. I think that he is very talented and that Scrubs is a great show.
Its just that Chevy Chase is Fletch. I know that he has fallen way off in the past few years, but I could not imagine anyone else playing that part...he IS Fletch. What makes Fletch great isn't just the dialogue, but the way that it is delivered, he made the charachter. It would be like having the new Rocky movie played by someone other than Sylvester Stallone, it just doesn't feel right.