The first answer was the best, Rigo. I don't agree with Tyson at all. The differentiating factor isn't the ability to beat someones ass using their fists- any boxer worth talking about can do that- but rather the method in which they do so in regards to footwork, angles, and distance.
That's why I don't like any of the aggressive, come forward guys as an answer. A lot of the most successful strikers in MMA are the cagier guys who make you come to them. not because they're necessarily better strikers, but by controlling the range and limiting the grappling exchanges they take place in they spend more time playing their money game.