It is really in the provided text: the movie was "ok-ish" and forgettable: a bit silly comedy about teenager killing vampires, something really hard to relate to. The TV show director has noticed its potential, found a ways to improve it and turned it into a well liked mix of a high-school drama, soap and horror.
As a TV show, Buffy The Vampire Slayer transcends its title: “Ignore the name,” fans would insist during the early years, while trying to coerce friends into taking a chance on a WB teen drama featuring monsters and a soap-opera alum. The movie, on the other hand, isn’t far from the glorified comedy sketch its title implies—like the later Beverly Hills Ninja, but with bloodsuckers instead of ninjas. To watch it is to gain new appreciation for the tonal balancing act Whedon [the creator of the TV show] achieved even during the rocky first season of the show [...]
In the movie, the horror is hokey: a mixture of stiff Victorian-era flashbacks that play like a parody of Hammer’s Dracula films and bloodless biting on the teen-vamp style of The Lost Boys. The high-school melodrama doesn’t scan much, either, because Buffy and her friends are vapid cartoons; [...]
On the big screen, Buffy The Vampire Slayer was exactly what it sounded like: a dopey comedy about a girl named Buffy who kills vampires. It was when it moved to TV that it became something deeper, something richer, something more: Buffy The Vampire Slayer And Friends. The “And Friends” is silent.