I wish I didn't have to only pick three attributes, because "Good Value", "On Time", "High Integrity", and "Creative" all apply too.
Aaron's CSS kung fu is second to none, and just in a few short months I learned more about CSS and HTML by looking at his code than I did in the past year creating hot prototypes and sites on my own at Yahoo. And I'm no slouch at it either.
The thing is, Aaron gets web design at a fundamental level, and in the way it was intended. He understands document semantics, and has such substantial experience that he knows the way to do things in the least amount of code, and the most efficient way possible given the ugly situation of IE and Firefox's vastly differing CSS implementations.
He also keeps at the bleeding edge with developers of each browser, and the W3C, so that he is never making design choices based on browser semantics five years ago, and so that he can tell each of the contributors to the CSS where to go from here.
To add to all that, he's an experienced and disciplined senior engineer that is consistent, timely, and straightforward at all times.
Aaron is rare in the field, and it was a true privilege to work as closely with him as I did. I hope to work with him again some day.