It helps to have watched some of Whedon's previous work. The man is great at creating real characters - genuine personas that the viewer cares deeply about, even the antagonists. The writing is witty, subtle, and plays on our expectations in ways that are surprising and enjoyable. There's very little cliche. The man's skilled at his craft - and I feel I can use the word without seeming silly. Other shows are produced, Whedon's are crafted.
I liked Buffy, was fond of Angel. Saw one episode of Firefly on DVD at a meeting of SF fans, and was instantly hooked. (It was War Stories.) Then I saw the musical episode of Buffy, and I just fell completely in love with Joss. Not erotic love, mind you, although I'd probably be up for some cuddling.