It has a very witty script (which is why so many quotable lines come from it). Also, it's a masterpiece of plotting: everything pays off (which is why McKee uses it for his script-writing seminars). And it's well photographed. And all the performances are worth savoring. When you consider it was essentially assembled by committee (the very opposite of an auteur approach), the results seem like a miracle. Of course, at a time when every Hollywood studio was putting out a film a week, it stands to reason that the law of big numbers would come into play and a great work would be produced, if only by accident.
I think the romance is well-done, but on the other hand that's hardly the only reason to like the film. It gets just about everything right, and it's damned entertaining to boot.