Apropos of a recent trip, a list of places I've been to... but only in the airport.
Denver - After attending a wedding in Santa Barbara in 1992, I flew home through Denver. Got off the plane, walked about three gates down, and got on the other plane. I have never been back to Denver (or Colorado for that matter), though I thought we might go through Denver for a family trip. Turned out it made more sense to start in Salt Lake City. Sorry, Denverinos!
Cincinnati (or Covington, Kentucky) - there were a few years where I mostly flew Delta, and these flights almost always required a change in the Cincinnati airport, which is actually in Covington, Kentucky. I've been to other places in Ohio, but not the Queen City. I don't think I've been in Kentucky since the last time I flew through this airport.
St. John's, Newfoundland - this was an unscheduled stop, as there was a medical emergency on a flight to Dublin. We spent five hours on the ground, as the baggage handlers wouldn't take the sick passenger's bags off until they didn't have any Air Canada baggage to deal with (I've often wondered how much baggage they had to deal with at 3 am). In any case, as much as I am interested in visiting Newfoundland, I am still spiteful enough that I won't do so by plane.
Frankfurt - a cheap(er) flight home from a trip to England went through Frankfurt. I can't say I got the greatest exposure to German culture during the three hours we were there, but we did get a reminder that if you order a sandwich, it's probably coming out on rye bread (the wife is not a fan). I would like to go back (to Frankfurt or Germany, doesn't matter), and at this rate may do so before I ever get back to Denver.
Istanbul - flew through here during the recent trip mentioned at the top. It's a new airport, and is massive. You can easily spend 20 minutes walking between gates, with only moving sidewalks to help you on your way. Not surprisingly, there are a multitude of shops and restaurants along the way where you can take a break.
I feel like Long Beach should get an honorable mention here, as I flew through there for a work trip to Anaheim, but only passed through in the shuttle van. I suppose I can list Dallas/Ft. Worth here for the same reason, depending on how much you want to differentiate DFW from Grapevine.