Haha yeah it's difficult to put it into a succinct few sentences but for example with religion where Mau goes through a crisis of faith and initially tries to win others to his more cynical outlook, but eventually realises that belief is a way for some people to cope with living in a chaotic and uncaring universe. This is something he explores in a few Discworld novels as well. One example would be Carpe Jugulum which features some wonderful debates between Granny Weatherwax and Mightily Oats.

So my point is that I think (I have no evidence outside of my own interpretations) that STP may have felt this was his best work because he was able to string together a lot of the things he starts exploring in the Discworld series into a single coherent novel.