I think road bikes are more comfortable than mountain or hybrid bikes. Having lots of different ways to keep myself upright on the bars has me moving around more during a ride. People go 100 miles on bikes with flat bars all the time ... and I used to do long rides on mine. But it's even better doing them on a road bike.
Also, a good road bike is a very well tuned machine: sleek and sexy. They handle much better than other types of bikes, taking corners with ease going down hills. I thought I'd be a little disappointed having one, giving up dirt trails ... but I think road bikes are generally more fun than other types of bikes.