Some of my favorite (not in a good way) query letters are the ones where an author attempts to show off his or her powerful grasp of the English language by cramming as many big words into their letter as humanly possible. The result ends up being something like this:
"Our protagonist, a multitudinously faceted lothario, possesses prodigious, grandiose idealism for an altrustic society in which humankind discredits its pedantic epistemological pursuit of ..."
You get the idea. I have a message for these authors:
Put the thesaurus down. Just put it down. You might hurt someone with that thing.
You must remember that we literary agents were (almost) all English majors at one time. We know the big words. And frankly, I'm very happy to have left those big words behind in college, where they belong. Reading these types of query letters takes me back to late nights writing papers on literary theory, and that's not a happy place. I'd much rather read a Steven King novel than Wittgenstein.
So choose the best words for your query letter (and your book), not the biggest or most obscure. It's a book, not a term paper!