The screenplay was written by Joss Whedon, but the film wasn't produced or directed by him; I think this makes a difference. Whedon is the American TV version of Kenneth Branagh; both have the ability to draw from actors a kind of transcendence. Unfortunately, in the movie Buffy, only Donald Sutherland (a wonderful pre-Giles) and, oddly enough, Luke Perry, seemed to have a clue how their characters should be played. Everyone else either hammed it up or played teen movie angst (á la Pretty in Pink). The point with Buffy is that it has to be played seriously humorous.
This means that first of all, the world of Buffy has to be accepted as absolutely real with real consequences. When the director said, "The movie isn't about vampires. That's just the milieu," I thought, "Lady, you have so missed the point." It is real. It has to be real. When, in the show, Buffy says to Jonathan, "My life sometimes sucks beyond the telling of it," you have to believe that she isn't just hamming it up. This is a world where teeny-boppy-dom meets eschatological world-dom and works.*
The solution isn't angst or hamminess. The solution is whimsy, the slide-by-and-miss-it humor that Whedon constantly employed. The movie has its moments but in general it feels surprisingly non-Whedon-like.
Second of all, Buffy has to stay Buffy. I gained an immense appreciation for Sarah Michelle Geller while watching the movie. Kristy Swanson starts out as a kind of Cordelia character, but as soon as she becomes a vampire slayer, she turns into tom-boy jock girl. And that isn't Buffy. The point with Buffy is that she never does turn into the proper image of the slayer (only in alternate universes). Not only does Buffy herself preserve her ultra-feminine Buffy-ness, her intrinsic personality is protected by Giles. I'm rewatching Season 2 right now, and I was struck by how, despite his many many complaints, Giles resisted turning Buffy into a friendless, fighting machine. It bothers him when she becomes obsessed. This, I think, is fantastically important. The reason this vampire slayer matters is because this time, the tale went different. (Despite my loathing for the new-age-women-have-power ending to the show, I think there was a kind of a metaphysical pay-off: Buffy got what she wanted--she got to be ordinary, one slayer amongst many.)
So why did I love the movie so much when I was young? Well, it’s a teeny-bopper film, and I was just out of my teens. But I think, too, that I loved it because it was early Whedon. After all, I hadn't been spoiled yet by the show and Angel and Firefly, etc. etc. When you don't have the real thing, you take what you can get.
*One of my favorite episodes with that transcendent, eschatological feel is the one where Cordelia wishes Buffy had never come to Sunnyvale. Her wish is granted by the to-be Anya. When Giles figures out how what has happened, he decides to smash Anya's amulet. She sneers at him: "How do you know that other world will be any better?"
"Because it has to be," Giles says, and I swear, I always tear-up at that point. There's so much pathos in Giles' desperation. And he is serious. Like I wrote, the Buffy universe must be taken seriously but not dead-seriously (ha ha), or rather not earnestly--yuck--which is why the whole new-age thing bugged me. It was so look look we're making a relevant statement about important stuff look! I hate that sort of thing. (And that was even before grad school.)
I think the difference between seriousness and earnestness is that when Giles says, "Because it has to be," he is talking within his world, his perspective. The viewer is satisfied, but it doesn't matter what the viewer thinks. It seemed like the end of the show got way too obsessed with what the viewer might be thinking and therefore became pitiably earnest.