Recently announced plans to reboot late-90s TV show Buffy the Vampire Slayer without director/writer Joss Whedon have thrown devoted fans into a tizzy. I think it's a bad idea, but not for all the usual reasons.
First, Buffy was very well-received in its original run, which is recent enough to remain in many potential viewers' minds. Recent popular reboots have generally rescued previously lackluster franchises like Batman, not tried to follow up shows that are already making top ten lists. The subject matter (vampire teen drama) is also a declining fad. As a result, the movie will be in for a lot of undeserved criticism even if it is excellent - not the best place for a commercial venture.
Second, the best parts of Buffy were either expert iterations of the show's core premise - "high school meets horror with a strong heroine" - or experimentations with the form. The first category lends itself very well to an iterative format where the characters develop but the plot doesn't - i.e. a TV show, not a two-hour movie. The second category is pretty much the exclusive province of serial narratives like TV, books or comics, where a sea of largely forgettable exposition can be cashed for great moments that require a prior investment in the series - the musical episode, the silent episode, the grief episode, the puppet episode, etc. To play with the movie form, you want an original IP, not something that has to stop and wink to the fans every 15 minutes.
Third is the cautionary tale of Buffy's slow slide into fantasy schlock. Like its cousins Harry Potter and Twilight, the show flounders almost immediately upon leaving behind the main setting, the main love triangle, and half the supporting cast. Further high points in the storyline all revolve around the dramatic return of characters from back when the show was good, now freed of their earlier plot shackles. The reboot is "not the same old high school Buffy" - but unfortunately, the true potential of Buffy was when she could be described in those terms. Past that, you're just putting Hellboy in a push-up bra and saying, "Hey guys! I crack wise and banish demons to the pits of Hell! Why doesn't everyone want to see this movie?!?"