I was thinking about why I love his books so, and for me it's because yes, they're fantastic entertainment and the product of a huge intelligence, but what gave them their quality was that you could always tell that he cared about human beings. There's such a strong sense of social justice and anger at those who treat people like interchangeable spare parts, or where a society is set up to benefit the system, not the people who live in it.
You also got the sense of a huge amount of research and knowledge - not that you ever got beaten over the head with it, but I loved spotting references to folklore, literature, film... and I'm sure I never even noticed half of them.
Humane, a wonderful writer and extremely funny. I'm going to miss him.