"That's all" is probably a somewhat.. selective statement.
Blue jeans, the reddish top, masculine physique and the blonde army-style hair from the newer games. In fact, I'd say all it's missing is the metal arm brace things and you might even call him Duke. Of course, Mr. Nukem DOES have a very generic, easy to stumble on design, but still... He's well known, so people will make the connection.
Do love the little guy, though. I really like the animations.