where Python sits in this ? Can't the Python community (or Ruby, ....) push computer science forwards?

Different communities (whether language centric or not) have their own reasons for forming. These reasons dictate the kinds of problems that they tend to work on and the kind of people that they tend to attract. My apologies if this seems self evident. The Smalltalk community has its roots in the PARC Learning Research Group, and I think that this has provided a center for it. The most comparable thing that I can think of in the Lisp community would be the Lisp Machine project, which turned into the LMI/Symbolics war, and was decapitated by the "failure" of commercial AI systems. As far as the Python and Ruby communities are concerned, I'm not aware of any large vision that is driving them. Python and Ruby are also dogged by the "scripting language" label, which causes many people to turn up their noses at the merits of what these languages have to offer.

Part of the answer is to look at a community and look at the problems they are trying to solve, and look for the projects that you think are pushing the state of the art forward. Kay said that it is hard to find people who want to argue about what's important in a way that produces forward motion. Look at the arguments the people are having, and what they are about. See if the way that the arguments take place is producing better ideas.

I think that one or more of the open source communities could/should evolve into such a place, but I don't believe that this has happened yet. I've haven't taken a close look at what the Squeak folks are up to, but I definitely plan to.