I totally agree with your first two points. I like the way Buffy and Giles slayer/watcher (father/daughter) relationship formed right from the start. And yes, it was good to establish that not all Buffyverse monsters are vampires!
The continuity is really bad in the first and second seasons, but I know those kinds of things can't always be avoided. Sometimes a character will have been brought in for one episode and end up becoming a reoccuring character (ahem, Spike) and therefore their original backstory needs to be fleshed out and expanded. Sometimes consistency gets lost in that process or the writers simply want to change something about what's already been established (like the whole issue with Spike's sire).