Rather than rehash a Snow Leopard review or for that matter even bother writing one up myself you should read the outstanding review at Ars Technica.
Ars super Snow Leopard review.
I will point out the three sections that if you are a developer you really want to read if going over their whole article is too much.
1. LLVM and Clang – Right now gcc keeps finding himself looking over his shoulder.
2. Blocks – Ruby, Javascript, and all the other modern scripting languages (and the not so modern like Lisp) welcome C to the party.
3. Grand Central Dispatch – Used with blocks this could be a mind blowing change to writing parallelized code. This is really wicked cool.
Oh, and Exchange support works with Mail.app and iCal. Like for real. Look I can even accept an invitation to play basketball without logging onto Outlook Web. *Swoon*
But you didn’t actually accept that invitation because you were too busy swooning!