A lot of people have been asking me the difference. I also think I learned something at Koshkenong yesterday...
Seat harnesses have a couple straps that loop under the leg in a criss-cross fashion to prevent the harness from "riding up" your ribcage. The power of the kite is also transferred more through your hips, instead of lower back, making your body's attachment point to the kite lower than with a waist harness, which would have the spreader bar more at your solarplexis.
When riding a seat harness, its more of a sitting feeling, and your center of gravity is higher, in my opinion, since you're kinda sitting up. Sure, you can lean back, but not when the kite is fully depowered. To depower the kite, you gotta let the bar out. When the bar is fully out with a seat, if you're not sitting up, you can't reach it as easily, unless you have long arms. This also forces you to use your legs more. When overpowered yesterday, my legs were burning holding the kite because I was in a more crouched position to be able to reach the bar. With a waist harness, I would have been able to lean back more with the hook higher on my body, and straighten my legs out a little more. This difference would be like the difference between squatting at the dinner table with no chair, or standing at the bar, eating your food. Having your legs bent at a 90 degree angle really forces you to use the leg muscles.
However, I think the lower attachment point of the seat harness helps on spins. because the weight is distributed more evenly, so it makes it easier to spin under the kite.
However, I think the lower attachment point of the seat hinders board-off tricks. By being hooked in lower at the hips, you have to exert a lot of effort to do a "crunch" (situp) to reach down to grab the board. With a waist harness, being attached higher, its easier to lift the legs to the chest to grab the board. I noticed a difference with loosening up the straps of the seat harness to let it ride up higher a bit.
I've got one of the new Felix Pivec waist harnesses from Prolimit coming in this week to check out the differences even more, since now I can actually tell, as opposed to two years ago, when I didn't know what the hell I was doing. This new wiast harness looks really good since the inside of the harness is molded to fit the shape of your back and gives extra room for the spine. It looks way better than the DaKine equipment. So I think I will be pretty comfortable in this, even though I usually get a sore lower back riding a waist harness. But if anyone has any additional comments, let's hear them!