Fun, engaging characters. Pretty well developed universe. And occasionally, both character and idea wise, they go deep. Stargate is aces in my book.
When I really want deep, I go for BSG or Babylon 5. I want light but still smart, I go Stargate. When I want a show that rides both lines regularly, Trek or Doctor Who. Or Farscape, when I want sexy and warped with the previous.
And Firefly, for something completely different.