Nothing.
Great car, definitely very comfortable.
Great for the highway, maybe a little big for the city.
Yes, the 78 cougar absolutely is a fantastic car for long road trips. I had a 78 XR7 351W that I drove from Philadelphia, PA to Flint, Michigan to Houston, TX, all the legs of the trip straight through and she didn't even cough. In Texas I was cruising @ 75 MPH in 100 degree heat with the air conditioning running and the Cougar didn't even make a move to overheat. The car had about 90K miles on it at the time. The Cougar is an absolutely bulletproof car.