Sam,
I broke by back in several places a few years ago and after two extensive reconstruction surgeries, I live with a collection of pedicle screws and rods holding things together.
A few design features can make a big difference for my back problems. For example, sitting bolt upright with my back at a ninety degree angle to my thighs is uncomfortable. So I look for a back rest that can be adjusted, or legs whose length can be changed to give a bit of incline. Second, a mesh covering over the entire seating surface is more comfortable than hard bands or rigid members.
As for specific seats that work with my back problems here is what I've found work best. They are listed in no particular order.
Waldrop pac seat. Can be used free standing or leaned against tree. Seating area is all fine weave nylon mesh. Very comfortable and light. It sets close to the ground, and I've seen a few instances when I ran into problems with bows 64" or longer.
Millennium strap on tree seat. Light with all seating surfaces covered with nylon mesh. Works great on trees from six to 18+" in diameter. No way to adjust incline unless you find the 'right' tree.
Millennium G100. This is a brand new model for 2014. It is their tree seat that has a modified frame that mounts on a swiveling tripod. Light, at around 7 pounds. Probably the best all around free standing seat I've found. Easy to carry and one step assembly. By changing the leg length I can get a reclining angle and sit comfortably for several hours.