It's dated in the same way anything becomes dated a few years after it's release - fads, clothing, technology, fashion, ect. all change very rapidly. What keeps something timeless is when it's able to transcend it's era. For example, I love the movie They Live (1988). It's full of late 80's fashion and style, but the plot is a timeless message, the characters and acting are all very well done. It doesn't matter if the story took place in 1988, or 2008, or 1958 - it's the story that I watch it for, not the time period it's set in. I feel the same way about Buffy - the earlier seasons have a very strong 90's feel, but the characters, story, and action are what keeps me coming back to it, not the time that it happened to be filmed.