I'm not a big fan of nature shows, but Planet Earth on the Discovery Channel is a really cool series. Probably one of the best uses of HD I have ever seen.
Another show I really like is Good Eats on the Food Network. Alton Brown entertains and teachs. And I like all the fun camera angles the show uses.
I'm trying to use tv as an educational tool for myself. I even watch about an hour of Spanish programming a day just to learn the language. And to see hot women and soccer highlights. I know I'm watching too much Univision and Telefutura because I see the Paulina Rubio ProActiv Solution commercials much more than the Jessica Simpson or Kelly Clarkson one.