I started watching Buffy when I was in 8th grade during the show's third season; I actually don't even remember the first episode I saw. I do remember watching Graduation Day Part II and explaining the characters to my mom ("What do you mean he's a werewolf? lol).
The day Once More with Feeling premiered, I was a couple of weeks shy of my 15th birthday and I made my mom watch it with me (I still wasn't used to having cable in my bedroom, having gotten it in the aftermath of 9/11, so my mom wouldn't have to watch shows like Buffy with me). One of the things that sticks out for me is that I didn't bat an eye at woman singing a love song to another woman--I'm not entirely certain that I actually understood that what Willow and Tara had wasn't entirely normal and I may have thought that they were just really good friends (I was a little naive). I think that Buffy is one of the reasons that I was able to accept that gay people aren't this huge threat to society. Willow and Tara's relationship developed so naturally and there wasn't a big deal placed on the fact that they were gay, so they were just there, a part of Buffy's crew.
One of the reasons I love this scene is that Tara is unaware of the fact that she IS under Willow's spell (in the prior episode she and Willow fought over Willow's over reliance on magic and Willow did a spell to make her forget about the fight). I doubt that I got the irony when I was 14, but I do now and I appreciate the song all the more for it.