One of my pet peeves is being forced to take a train, i.e., having no option to walk. For this I like Atlanta (you can walk parallel to the train tracks connecting the concourses), but dislike Denver. Unfortunately, many airports don't have a walkway -- Las Vegas, Dulles (of course, the "people movers" are a whole separate abomination), DFW (I think), Pittsburgh -- but they should. If I have time I would rather walk every time.