In apperance, in style of character, everything. See Wayne Knight had experience playing a despicable, arrogant unlikable douchebag on "Seinfeld." I believe he could channel that same energy into "Harry Potter" as well, much better. Also the apperance would have counted. After all, the English surname Dudley is of Anglo-Saxon origin. Knight has Scandinavian features (small eyes, thin eyebrows at most). Plus the comedic energy Knight displayed on "Seinfeld" playing a despicable Klansman-type of character, he could put into Vernon.