I'm subscribed to a couple of constraint solver mailing lists. A common question is "why is my constraint solver doing this...?", or "how do I massage my constraints to achieve this...?"
With CSS You can't build a layout that 'fails' (in the sense that a solution cannot be computed due to improperly defined dependencies), and you certainly can't send your browser into an infinite loop by applying an incorrect style.
I still really like the idea of constraint based systems, but I think they are more suited to (web)applications rather than documents.
Which is what some people will tell you web is about when you point out to them they're doing something weird instead of writing a document communicating a message. Maybe it's time to separate those two use cases - web pages and web applications - because otherwise people will keep getting confused between the needs and problems of the two?
Anyway, GSS looks pretty cool.
For people that are use to hacks + grid systems, it seems weird at first, but over time you will become familiar with a rich layout system and stop thinking left-to-right, it will become more natural. The language of the spec and the declarations (align/justify distinctions, etc) can be weird at first, but you get used to it.