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.