Well hell, I never saw any of the episodes after the Kyoto Arch, so to me, it ended right there. I remember watching this years ago, and considering I still remember just about everything now, it just shows how good this show is. Yes, the OVAs do add a lot, and they're as close to perfect as you're going to get.
But basically why I like this show boils down to only a few things:
1) Kenshin himself: I absolutely love his character; he's probably my favorite character out of any anime.
2) The fights: they were dragged out and slowed down by too much talk like other shows (cough-cough - DBZ) They were extremely fast paced and intense.
3) The setting and plot: I'm a history buff, so to finally find a show that is actually based (however loosely) on actual events is just awesome.
4) Saito: He just kicks ass, plain and simple.