In discussions with a charming 11-year old over the holiday, I mentioned the truth of Mark Twain's observation that "the difference between the right word and almost the right word in the difference between lightening and a lightening (oops, lightning) bug."
I also told her I liked to collect words that mean precisely what they intend and that express something no other single word can convey. Interstice is a favorite, and liminal is another. The list also includes interregnum, and a friend suggests matriculate (which, like penultimate, also has the distinction of being misused more often than it is used correctly).
Any other nominations or arguments?