I was lurking in the youth section of the library (that's where all the Buffy books are kept.  Grrr.) and found the Candace Havens sort of biography re Joss Whedon, so I thought I'd boil down the writer type things that Joss does that make Buffy so Buffy.  There wasn't all that much, so I stuck in some other stuff, too.

1.  Change your name to a foreign word that means Good Luck, preferably Chinese, but Joss is already taken.
2.  Never give the audience what it wants.  We've all heard this one.
3.  Undercut sentiment with a joke.
4.  Develop the character of the sidekicks; don't focus exclusively on the hero.
5.  Combine all four genres - horror, comedy, action, drama.  (Not sure that JW actually gets real horror in all that often.)
6.  Take your most humiliating experience (or that of the writers in your team) and use that.
7.  Manichiastic character development - what is good becomes bad; what is bad becomes good; or kill it off. (Then bring it back.)
8.  Steal, steal, steal.

You forgot one.  Torture your characters and never give them anything resembling happiness.

Hee!! Love your reasons, especially steal, steal, steal.

There's also: No relationship ends happily. Never.

Niamh and Sosa - those both come under never give the audience what they want - :)

Also, one-shot or minor characters can become important/annoying later.  See:  Spike, Warren, Amy, Johnathan, Riley, Amanda.  Probably a few more I've forgotten.