This started in response to the comments on this Cary Tennis letter but got far too complicated for a comment thread.
Think of the pool of all possible conversation topics - everything you might ever conceivably blurt out - as a well-organized email folder system. The vast majority of the emails are archived by topic. These are things you can say in reply to productive and substantive inquiries. There are a few emails in your inbox. These are new things that you can introduce during a lull in conversation. And there's a bunch of crap in your spam folder. These are things that are completely useless in conversation. (e.g. "There are four light switches in this room." "The capital of Uruguay is Montevideo".) You hardly ever look in your spam folder anyway, it's all the Nigerian finance minister trying to enlarge your penis and sell you fake university degrees anyway. Sometimes you do go into your spam folder for a specific reason, just like sometimes you do need to know what the capital of Uruguay is, but the vast majority of the time you ignore it and it isn't even worth thinking about.
I think introverts have a stricter spam filter than extroverts. We have things in our spam folder that more extroverted people would consider suitable conversational openings. There are fewer things in our inbox, and some of the things that (by conventional social standards) should be in our inbox are in our spam folder.
For example, it would never ever in my life occur to me to ask a casual acquaintance about their vacation plans. The topic was simply in my spam folder, right in between "I have a hole in my sock" and "I had two cups of coffee today." (Yes, these are things I might just announce to a close friend, but, as I've blogged about before, it works differently for close friends.) When I read someone mention that as a possible topic of conversation in the Cary Tennis comments, a lightbulb went off. "Oh, THAT'S why people at work keep asking me that!" Because it was in my spam folder, I figured they were asking me for a particular reason, just like if your best friend sent you a penis enlargement email you'd assume they have some particular reason for doing so.
So where extroverts can just reach into their inbox - the first page you get to in any email interface - to find an appropriate topic, our inboxes don't have enough topics. So first we have to come up with the idea of looking in our spam folder at all. Then we have to sort through it trying to find something that's less crap. We can't give them just anything from the spam folder, we have to sort through the whole thing (and how much crap is in your spam folder right now?) trying to find the conversation equivalent of, say, a shoe sale flyer rather than penis enlargement spam.
And the other problem is, once we find the exact conversational nugget we need in our spam folder, we think "Hey, there's some useful stuff in here, let's filter less strictly so it ends up in the inbox!" Then we set our spam filter too low and end up with all kinds of crap in our inbox, and the next think you know we're walking around offering to enlarge people's penises. This manifests itself in the phenomenon of people who claim to be introverts going off on a babbling rant about themselves or their interests. Because all the stuff in our inbox tends to be stuff we're genuinely interested in, if someone treats one of our spam topics like an inbox topic we assume they're genuinely interested.
So unless you want us randomly free-associating and dumping the entire contents of our mental spam folder on you, you'll have to either tolerate our pauses or take more than your share of the lead.
Edited to add: Having been bullied adds another dimension to all this. My bullies would often ask me questions that would sound perfectly innocuous to outsiders and that adults with benign intentions may well use as fodder for small-talk, but the bullies would use whatever I answered as fodder for bullying.
For example, they might ask me what I did that past weekend. If I didn't do much of anything (which, objectively and outside the bell jar of adolescence, I rather quite enjoy), they'd mock me for not having any friends. If I did something with my family, they'd mock me for spending time with family because I don't have any friends. If I did something with friends, they'd mock me for the insufficient coolness of my friends or our activity. In the weird world of middle school, it was a loaded question to which every possible answer was socially unacceptable.
So because of all this, a bunch of topics that appear benign to outsiders are quarantined in my mental spam folder because they look just like emails that have previously given me viruses. After having been judged so often for my answer to "What did you do this weekend?" I wouldn't dare ask that of an acquaintance or co-worker any more than I would ask them "So are you a top or a bottom?"