Are you going to read the 60 or so original comics that were published during the course of the show? They cover the time between the movie and the series, the one with Dawn and Hoopy bear is especially good and we see what happened to Pike and much more of Joyce and Hank
Yeah, unfortunately Jesse was that TV/movie cliche, the character who appears one week, means the world to everyone, is killed giving everyone angst and motivation and then never mentioned again. Shame he couldn't have appeared to Xander in Conversations with Dead People.
As for Riff Regan in the Buffy pilot it was a shame but she came across as a realisitic nerd. Unfortunately for her but fortunately for the series she was replaced by a girl once voted 11th sexiest in the world by FHM (much like Glee, the Scoobs are an amazingly attractive bunch of 'outcasts')
I liked Flutie and he did care, he's very similar to the Principal in the movie but Snyder is a necessary protaganist. Giles is the cliche but he has to be for him to loosen up later.
As for James Spader, well in Get It Done we discover why Slayers have a yen for the kinky. As for CC and her Com-shuk-me vibe, I'm just not sure?
One thing, can you see how much the Hemery High characters in the movie resemble those in the series? (and still might if after the end of Chosen Buffy wakes up sane again in the asylum, meaning they're all still alive?)