Hey, thanks for the feedback! I'm the lead designer on this project. Gravity probably won't replace Google Reader just yet. Out goal was to create a fun experience, show off a little bit of our network, and listen to feedback – feedback that we might use to drive the design and features of potential future Disqus products.
This is very different than most of the stuff I've seen using d3. Do you have plans to "disqus" how it was done?