Emma Smith disagrees with you. On chapter 9 of her book she says that shakespeare has great villains: R3, Iago, Edmond in KL whereas Don Jon is a "wooden" character and not Keanu, that shakespeare phoned it in on this one, and Keanu's portrait is as good as it gets.
EDIT: I think Emma , with all reverence she deserves, is wrong, and despite liking Keanu in his traditional roles (Matrix et al) and the fact that he is apparently a cool guy. It is possible, as it always is, that it is the director's fault, either bad directing - or no directing.