We were diverted to Gander in Newfoundland a couple of years ago on an American Airlines flight from JFK, and that was due to a 'smell of fumes' in the cabin. Turning back 2 hours with that is not fun by any means!
We also once had a flight diverted to Washington on the way to Atlanta, and that was for a medical emergency, but even though the sick person was off the flight in minutes, we were grounded for hours because we then needed to refuel, and you need all the checks and certificates done again before you can take off.
Unfortunately, it is one of those things that can happen to airline, whatever class you are in, but the knock on effect is huge!