I have two main reasons. First, the characters. Who cares about plot holes when you get engaging character arcs for each character? They're so well-crafted, consistent, and well-acted.
Next would be the outstanding episodes. Episodes that got me hooked, like OMWF, Becoming, Hush, FFL, CWDP, and so much more.
Can I add a third reason that doesn't pertain to the show itself? The fandom. I watched Buffy long after it aired, only last year. My interest would have faded with no one to talk to, new viewpoints to consider--and I found the fandom still active after ten years, on Tumblr, LJ, and most of all, here on BB. It's awesome.