I recently re-watched the movie, for the first time in years, and, while it certainly had its problems, it wasn't nearly as bad as I remembered it being. Comparing it to Joss's script, I was surprised to see that the majority of his stuff was still in there.
The biggest problems were:
1. Donald Sutherland was an asshat and kept re-writing his lines and re-defining his character to suit his own taste. The producers were so eager to keep their big-name star happy that they let him get away with anything. All the nonsense about Merrick being an immortal being, the eternal "Watcher" who kept being re-born into every generation was his idea. They also changed his death scene all around.
2. Rutger Hauer's acting was terrible. I liked him in Blade Runner and Ladyhawke, but he was just terrible in this. Apparently he was also trying to hit on Kristy Swanson the whole time.
3. The deaths of the two main vampires -- Amilyn and Lothos -- were changed from the original script into much lamer, pretty much comical scenes. In the script, Amilyn was tricked by Buffy into thinking the sun was up (Joss recycled this in "The Harvest," where Buffy uses the same trick on Luke). In the movie, Buffy just stakes him and he does a comical Pee-Wee Herman style death scene.
In the script, Lothos had Buffy trapped against a row of lockers, and she grabbed a pencil out of one of them and staked him with it. In the movie, Lothos looms over Buffy with a dagger (I think), raises it two-handed over his head, and then pauses dramatically long enough for Buffy to kill him with the stake she was holding right there in her hand. In other words, this centuries-old Vampire King suddenly became so stupid that a five-year-old could have killed him.
Those were the big issues. But it wasn't all bad. Kristy Swanson and Luke Perry weren't nearly as bad as I thought I remembered. Stephen Root was hilarious as Gary Murray, the guidance counselor whose lectures keep veering off into reminiscing about his youth as a hippie stoner. The final conversation in the gym between Luke Perry and David Arquette's characters actually worked better than the one in Joss's script, I hate to admit.
If they'd avoided messing around with Merrick's character, left Amilyn and Lothos's death scenes the way Joss wrote them, and hired a better actor to play Lothos, it could actually have been a good movie.
