How's this for an answer...
When I discovered some of his original concept art on display at the Star Wars Celebration convention here in Los Angeles back in 2007, it was such an exciting nerd moment for me, I actually called my mom to tell her what I was looking at. She above many knew what a huge artistic icon he was for me growing up. My brother, who was there, thought this so funny and unimaginably dorky that he captured the moment on his camera phone.
The art was amazingly small. Making it all the more impressive. What I marveled at most was how clean his use of paint was. Nothing seemed sketchy, loose, random, or corrected later. His skill as an draftsman was evident as many of the finer implied technical details on spacecraft and such were just dark, ruled in, pencil lines on top of the paint. I loved that you could even catch light reflecting off the graphite at certain angles. It really brought home the fact that it was the actual painting in front of you.
Out of McQuarrie's entire body of work, this one is my favorite. It captured my imagination when I first saw it in '77 and still does today.