Its an issue of campaign scaling. If I create a world where you advance at a snail's pace, and so the really big badass kicking your tail around is "only" 8th level, it really doesn't matter because you're only 2nd. Buffyverse style magic users need a new (or heavily tweaked) system to make them viable- they can do some powerful things, but are nearly useless in combat (unless juiced to the eyeballs with dark magic- a story event rather than a level-traking issue in my mind).
It may involve tweaking the DC scales on certain tasks to bring things down to scale, but this is hardly rules breaking.
Also, the advantage of running a very low-level slow-advance game is that things like +2 Strength or +1 to Hit become fairly serious advantages for the long term, and they don't get 'lost' into the mix as you advance in level.
One thing I was considering-
Use only basic classes.
Everyone picks a Template which maps to a buffyverse role:
Vampire
Slayer
Witch
Zeppo
Watcher
etc
You advance along whichever basic class fits your character the best (BUffy- Strong, Fast,; Spike- Charismatic; Tough; Willow- Smart & dedicated or whatever)
Your template gives you upfront advantages and disadvantags (enhanced strength & speed, healing factor, magical ability) and your ability scores, level, skills, and feats determine how good you are at your template's set of goodies.
Template skills would cost the same as class skills, and each template would have a set of special feats unique to it...
Using massive damage rules ofr vamp-dusting is a very very good idea... I mean, a vamp is no more vulnerable to destruction than a person when it comes to weapons- a stake in the heart, or decapitation, or being burned alive will all pretty much kill just about anyone... Basicly, a vamp's classic weaknesses are more of an advantage- he NEVER has to save vs massive damage against anythign except those few things which can kill him.
Thses templates would really be like level-equilivent races, but when everyone gets to pick one at start-of-play (or later- if a zeppo gets vamped for example). Its a wash- no one has the special advantage, and it makes for easy-mook creation. Just apply that template to a lvl 1 mook.
I think it would be quite a fun game to play out.
-Ben