I agree; Denver's exhibit is clearly a child of the 80s but it is far away from being the worst I've seen (and having recently returned from my visit to the Baltic countries, I don't think I'll ever see as sad polar bear exhibits as I saw there again ... I sure hope not at least)
I have always been displeased with Polar Bear exhibit. I don't know if it would be possibly, depending on the underground area of the cage, but do you think some zoos could take a hammer to the concrete and grow some grass. It seems Polar Bear exhibit always hit the sun, mostly because there isn't any trees, with the right amount of depth and soil, I'm sure they could grow some grass.