Momentum.....sit on the wheel of a geared bike and draft with them on a fixie, you'll see what I mean. The momentum of the direct drive drivetrain really decreses the effort...you get sucked right up very different than on a freewheeled bike.

For example. I ride with a cycling team a few times a month. Until the past 2 times it was always on my road bike (like they all have). Sitting in a paceline or drafting with someone I could watch my RPM and heart rate and feel the effort I was putting out to stay at a certian pace. The last two times I rode with them I was on my fixie. I saw a dramatic drop in my effort and HR and thus my increased endurance for the ride. Granted I got my ass kicked going up some of the steeper hills and it was hard staying with the pack on descents (coasting at 35mph is WAY easier than spinning at 35mph)....but on flats, pacelines and longer more mild climbs....it was MUCH more easy...