Timing, Timing, Timing. Great timing. Cruise and Diaz had great chemistry, their comedic timing was excellent, action sequences melded well with CGI, and the camera work was top notch. Great escapist fantasy for the summer. It did not disappoint (please try not to compare it to that product placement dullard, 'Killers'). Cruise's facial expressions alone make the movie, but if you add to that Diaz's reactions, and the seamless action sequences, you find yourself leaving the theater with a sense of satisfaction. Pure escapism.
I would love to see some spy thriller crossovers. Let's say Mr. and Mrs. Smith cross paths in a diner with Knight and Day right when Jason Borne comes out of the bathroom. While they don't acknowledge each other, you still have some tension due to their spy tingling senses. Basically you have three stand alone movies peppered with these little happenstance meetings. I think J.J. Abrams would do a great job on that, given his penchant for subtle clues.