To me his playing is lyrical and melodious - quick lines, tasteful chords, clear melodies. I don't think it gets any better than this:
One of my short term goals is to play some classic Joe Pass interpretations of standard tunes (April in Paris, How High the Moon, All the Things You Are, Autumn Leaves, etc). In the meantime I'm developing my ear by listening to Unforgettable over and over again...
I should also note that I discovered Joe Pass by way of Ella Fitzgerald and while I don't really have aspirations to comp with a vocalist right now I'm not sure anybody's ever done jazz better than Ella and Joe together. Definitely worth a listen - Pass Again is really good and I've got some singles from other albums as well - anything by these two greats would be worth a listen.