The original Buffy movie was based on a script that Whedon wrote. The original script was closer to the TV show than to what eventually got made. At the time, Hollywood didn't see a lot of marketability in the idea of a badass, strong female character. So instead they made a parody of both horror movies and girl movies. Instead of grrl power, we got... a very mediocre movie. They brought in script doctors and a director and the end result was a very different Buffy. It came out back in '92, I believe. Whedon fans generally pretend the movie never happened.
At first, I, too, was excited by the prospect of a Buffy movie. Then I, too, saw that it's by the guys who made the original. Fuck it, ain't gonna see it, not unless Joss gives it a thumbs up.