The Humor:
Kristy is the star, and deserves to be. She handles the role perfectly. While many people have accused her Buffy as being less intelligent than the series version (played wonderfully by Sarah Michelle Gellar) remember that the movie Buffy had to start out vapid, but she has a nice arc: ending up serious and focused. Kristy Swanson can handle all the emotional levels of her character while delivering comic lines perfectly, and she does the physical stuff well, too.
Donald Sutherland. What else needs to be said? The guy's terrific, and remains so in this film. While many have criticized his performance as 'sleepwalking through the role' I think they've missed the subtleties in it. Sutherland has described Merrick as being 'in slow motion'. He has lived through 100 lifetimes, and so everything takes on a different significance to him. I love his distracted moments like tugging on the handkerchief with his mouth, and staring at the spinning teabag. These moments reveal a little about the eternal nature of the character just in his expression.
Rutger Hauer, star of genre classics like Blade Runner, has little, unfortunately, to do here. He gives Lothos some dignity and creepiness but the character is poorly defined. He gives it his best, though.
The role of Benny is played by the wonderful David Arquette. He gives a typically memorable performance - portraying a quirky, mischievous, and funny vampire. He isn't in the movie much, but his character sticks out in the memory.
Although Luke Perry has been put down for his peformance by almost every single critic, I feel his was perhaps the best performance in the movie. He delivers every line with a calm but purposeful rhythm, similar to Seth Green's Oz character in the series. Each word he speaks is timed perfectly to humorous or dramatic (and often both) effect. His relationship with Kristy's Buffy works very well, and is totally believable.
I also, however, love the movie's score, by Carter Burwell. He refuses to acknowledge the campier aspects of the movie, and reinforces the dramatic themes. In fact, I may go so far as to say Burwell saves the movie from Kuzui's excesses. (Discuss.) The scenes with Lothos and Amilyn are backed up by a wonderful (synthesized?) chanting that adds to the mythic and eerie nature of the characters. His haunting tune as Buffy lies into Lothos' arms is also moving. (The music here, and in other places, 'wobbles' on my tape - is this specific to my tape or a world-wide phenomenon?)