I think the aspects of a business that don't scale are how user behavior evolves over time. It takes awhile for groups of people to try new things and figure o

I think the aspects of a business that don't scale are how user behavior evolves over time. It takes awhile for groups of people to try new things and figure out all the different/awesome ways to use something (Twitter and @replies, RTs, etc are one clear example). There is some limiting rate for iteration...