I'm watching Buffy right now. The Cincinnati library system doesn't carry DVDs of older TV shows, but I've discovered the amazing Hulu.
One of the things I like about Buffy is that there is a full cast of minor characters--a handful of characters that play a part in one episode, and then actually show up in later episodes (often very briefly), and even in later seasons.
Deep Space Nine also had that going for it. Cool main characters, yes, but also very cool secondary characters. They gave the show a certain depth. It seemed more real because of them.
Compare that to, say, Lost. How many people survived the plane crash? And yet each episode seems to see a brand new set of minor characters. I understand it's difficult to keep that many actors around, just to play nonspeaking roles in the background. But still, with at least twice as many actors as actual survivors, and with almost certain knowledge that any plane survivor that wasn't a main character was eventually going to die, my disbelief had a hard time being suspended.