Neil Stephenson's vocabulary is astounding. He has used more words that I didn't know than even A. S. Byatt did, and she used all sorts of words from British literary academia. Some of Stephenson's words are archaic or rarely used in a modern setting or are simply trade/science/jargon.
The only difference is that I can usually figure out the meanings of those new words from its context from Stephenson's work. Byatt is much more troublesome. I still enjoyed Byatt, but I enjoyed her more after I threw the dictionary away from me and stopped trying to look up every word I couldn't figure out.
What novels or non-fiction have you enjoyed that you felt you had to chew through sometimes to get through it?