IAD: Good airport, bad Thrifty experience
On my last Thrifty rental at IAD, there was only one vehicle available - a full size van. It was parked way away from the other vehicles, so I had to walk the full length of the garage to get into it. It also burned a lot more gas than the economy car I had rented.
When I returned it, the Thrifty employee checking-in attempted to stick me with some damage charges. I had to whip out my digital camera and show the pictures I had taken of the van when I picked it up.
The good experiences that the OP had at IAD were because the airport required Thrifty to be in the rental car garage, alongside all the other rental car agencies.