So I'm truly curious what's at work behind this inability or unwillingness to talk to "the masses", as it were.Class consciousness. The problem has nothing to do with the content of the conversation, but everything to do with the subtext: You're interacting with someone from a completely different social group and you may not even be sure what social cues you want to send, let alone exactly how to send them.
What is the proper relationship between you and your plumber? Master and servant? Master and slave? Contract employer and contract employee? Genial boss and long-term employee? Two friends trading with each other? Fellow members of a communal society? Friendly rivalry? Unfriendly rivalry?
How does this change if the plumber turns out to be the cousin of your sister-in-law? If the plumber belongs to your church? If the plumber lives next door?
If the plumber recommends an electrician friend, how important is it to hire that friend? Why is the plumber telling you this tale about the draconian local house inspector -- is this a standard plumber gripe, the plumbing equivalent of complaining about the weather, or is he trying to excuse the fact that his work might not pass inspection? When the plumber asks to be paid up-front, is that a standard practice or could he be planning to get drunk and then skip town?
(Regarding that last one: The books suggest that you never pay construction subcontractors in advance, but only as the work is completed. Unfortunately, the books don't tell you the precise words to use to inform your subcon that, no, you were not born yesterday. That part is up to you, and it's one of those things that they can't teach in an Ivy League class.)
Obviously, the problem of conversation has nothing to do with the topic or your knowledge of it. Thanks to the web, we know that people can happily spend hours discussing topics that they know absolutely nothing about. And there are probably very few topics that you're afraid to discuss with your peers, because you know how to navigate their social world: you know the slang, you know the legends, you know the taboos, you know when you're being insulted and how to give insult in return, and you know how to change the subject. Most importantly, you know the social order. If I log on here and write "Arc is Blub", everyone knows that I'm either issuing a (crude) challenge or being a clueless noob -- the authors of Arc have high social status here. If I write "VBScript is Blub", everyone knows that it's the news.yc/Slashdot/Digg equivalent of talking about the weather. ;)