Yes, I love his shows: Buffy the Vampire Slayer, Firefly, Serenity, and now (!) The Avengers.
So, what can Joss teach us about Dialogue? Lots, but Firebrain from Absolute Writes has a great article on Making Your Characters Talk. (caveat--It's a bit raw) at literotica
Here are the highlights:
First Things First - basic grammar and punctuation for dialogue.
The Ellipsis and the Dash - you think you can use these bitches - apparently, you can't.
Dialect and Slang - because the poshest BBC accent is a dialect and even robots say fuck.
Utterances - learn to balance your erms and arghs.
Formal vs. Natural speech - because you're writing Joe Average, not Stephen Hawking (and if you are writing Stephen Hawking, I'm backing away right now).
Dialogue Tags - who says what and how do they say it?
Dialogue During Sex - should actually be shexy, and not in the Anchorman way.