The narrative structure is often wackily non-linear (which I like), the humor is often pretty smart (which I like), the writers show a sly awareness of the medium (which I adore), and there's a childlike joy to the whole thing that makes me happy. It has a sort of Calvin and Hobbes sort of fun to it, though I can't explain how.
Plus, you know, Alison Hannigan: hotness personified, and great comic timing. Plus, you know, Neil Patrick Harris: geeky coolness personified, and amazing comic timing.
Such a great show! I am flabbergasted!