What he said. A lightweight, flexible, high thread count casing with a relatively thin tread is almost guaranteed to give a nice ride "feel" and low rolling resistance. The casing and tread also needs to be straight, which seems obvious but is often imperfect. I've also found that natural materials (cotton, silk) feel much better than the modern substitutes. I haven't yet ridden a nylon tire (most of them are now) that felt as nice as cotton, let alone silk. The nylon is almost certainly stronger than cotton, though, and both are much stronger and more puncture resistant than silk -- although neither can match the ride of silk.

The main trouble, in my experience, is that the factors that give great "feel" also lead to more fragile tires -- the Grand Bois Cypres being a perfect example. The other end of the scale might be a tire like the Paselas and Rolly-Pollys that last forever and rarely flat, but also ride like ****.

I just ditched the entire debate and bought a pair of handmade 27mm cotton tubulars. Hopefully these are as good as I remember them from 20 years ago -- when they cost one quarter of what I paid for them last week...