I would agreed whedonesque stuff can be irksome, its spilled out into the MCU, even leaked into the last starwars. For me it is because the novelty has worn off, a tense moment which is undercut by a villain being humorous has lost its impact now. I do think buffy is more than that though, also while whedon was the creator and oversaw it, there were many longterm writers on it (Jane Epenson was my favourite).
I dont think it is fair to criticise it on character development, comparable shows would just reset the format every week, in buffy people grew and changed from their experiences. arguably it was influential in bringing in season long story arcs (sure people will argue twin peaks but that made it up on the fly, or the x-files that dabbled in arcs but not really), so is responsible for this golden age of television.
Think its ultimate strength though was the emotional weight it had, it was a fantasy show but was really about people, emotions and difficult situations, everything does that now and with hindsight it seems obvious but it was innovative and it did it well.
combined some genuine frights, with humour and relatable emotion. 10/5 best program ever