Not knowing the social conventions.
What is it that people are doing at parties? What are they talking about? Why are they doing this? It seems incomprehensible. With someone I haven't met or barely know, it's hard to find a way to say what comes to mind in a way that a total stranger would relate to. With someone I know well, I can say what comes to mind, because there's a lot of shared context. This is the common pattern among introverts: quiet with strangers, endlessly talkative with close friends. And why is that? Because with close friends, you have the shared context and conventions needed to have the kind of conversation that makes sense to you.(Please, no pity or advice. I've worked on my shyness and gotten good enough at connecting with strangers to occasionally even pick up women on the street--a skill every hacker entrepreneur should have. I'm just pointing out that the psychologizing is wrong. The path out of shyness, at least for a lot of us geeks, is simply to learn to play the social game, not to address some sort of self-esteem issue.)