I've sat in just about every possible configuration of plane and seat, including these new "ultra-light" seats (which are on a number of United's planes). The seats are slimmer, but the problem is that they are also stiffer, and the material is both harder and less supportive than the standard seat.
I'm usually able to deal with just about any seating situation, but I found myself getting uncomfortable after 30-45 mins in the new seats, particularly my back. I actually had to consistently stretch and turn to mitigate the ache that started to form. The major reason is, I think, the fact that there is less support for your legs in front, leading to a "sliding forward off the seat" kind of situation where you have to put more effort into keeping yourself seated.
Anyway, seats vary, and old seats suck as much as anything else. However, selling the new seats as "better" or "more comfortable" is a load of c$#% that the airline industry no doubt has teams of advertisers selling through stories like this one.