On the one hand, a broad vocabulary is a delight in that you can get nearer the shade of meaning you want. On the other hand, too many people with a broad vocabulary use it to obfuscate their meaning from their interlocutors (Q.E.D.).
My master’s work in communication taught me about the varying levels of communication and the dangers of getting stuck at one. Fifteen years in journalism taught me that the point of mass communication is to communicate to the masses, and that means using common words in common ways. I still have quite a good vocabulary, but I’ve learned not to use it in most settings. I don’t feel in the least that I’ve dumbed down my conversation or my writing; rather, I’ve improved my clarity of reception.
Some folks at The New York Times put together a (PDF) list of the top 50 words their readers don’t seem to have a solid grasp of. The methodology is well explained in the article. Some I actually could define; others … not so much but I’d be likely to read over most of those words without going to a dictionary and let the context enlighten me. See how you fare.
A note to those who teach: pay particular attention to word number 13. That this is unknown by a large segment of some of the nation’s better-educated newspaper readers is a grave problem. Do your part with your students to fix it.