Frame geometry

Some of the new "comfort" or "endurance" bikes have taller head tubes, slacker head angle, more fork offset, and longer chanin stays.

This allows for a more upright seating position for those with less flexibility, slightly slower handling, longer wheelbase for more stability, and more verticle flex.

A number of these models are still quite race worthy, and the cervelo and specailized models are sometimes raced by thier pro teams on rougher courses.

Really it is an acknowledgement that the buyers of most high end bikes are more mature customers (who have more money) but don't have the flexibility of <30 racer boys.

Solanog said:

So what makes a bike "comfortable"? Is it a matter of angles or rations between tube sizes (top tube/seat tube) or both?

Ratio, yes, but more about the ratio between the top tube length and the head tube length. If you compare two bikes with the same head tube lengths, and one is more "comfort" oriented than the other, it will have a shorter top tube, so you are not so stretched out. (There are some other dimensions that influence things, but that's the general principle.)

"Comfort" bikes are also good for people with long legs and short torsos, because they can get a proper fit without having to resort to short stems.

That's "fit comfort"; the fork doesn't really affect that, it affects "ride comfort" more, the way the bike reacts to surface irregularities and damps out vibration.