Jerkass Dissonance: Joss Whedon hated the idea of another vampire romance for Buffy (he never wanted the first one either) and wanted Spike to get killed despite his popularity, as he was determined for vampires to remain villains.
Memetic Badass: The general opinion among BuffyFan Fiction writers. In most Buffy stories, demons fear all of the Scoobies, but somehow they believe Xander is the worst one to go up against. Why? Two reasons. The first is that while Buffy and Faith are Slayers, and Giles and Willow have magic, Xander is just a guy; the fact that he's put paid to so many demons despite being just a guy means he's got to be a Badass. The second reason is that while Giles and Buffy and Faith will kill a demon, Xander will make them wish he'd only killed them.
Yoko Oh No: Spike's "Yoko Factor" plan in Season 4 revolves around this. Spike sees a conflict brewing among the Scoobies and pushes just the right buttons to set it off. Notably, Spike himself notes to Adam during the episode that Yoko, for all she got blamed, didn't break up the Beatles; it was internal strife, much as he wasn't so much the cause of the Scoobies' breakup, just the catalyst that (deliberately) brought their grievances into plain view. It's ultimately subverted, however — while the Scoobies do have a vicious argument and falling out thanks largely to Spike's prodding, when tempers have cooled a bit they're savvy enough to realize that Spike's been manipulating them, approach either other more calmly about their recent tensions, and mend bridges.