After an extremely dark episode, which advanced the ongoing story arc a lot, we get something a little more standalone and less dark. It's a strange occasion when an episode about a monster that murders sick children can be described as "less dark." Anyhow, more after the break.
"So this isn't about you being afraid of hospitals 'cause your friend died and you wanna conjure up a monster that you can fight so you can save everybody and not feel so helpless?"
Although there are some valid uses of the term filler (particularly in the music industry and in classic Doctor Who), there really is no such thing as "filler" on a show like Buffy. Making a single episode of television involves a herculean effort of talent and logistic on the part of hundreds of people. And in a writers room like the one on Buffy, everybody's hands are on everyone's scripts, working hard to make each episode better. It's easy to forget this, because we can consume an episode in less than an hour and pay as much or as little attention as we like. There's not a single episode that was thrown together haphazardly to fill out a season order. The notion that they could do that even if they wanted to is absurd.
Which is not to say that every episode is worthy of high praise, or even particularly consequential. Not every episode is going to affect the viewer in the ways "Passion" did. "Killed by Death" certainly doesn't. But to call it "filler," to imply that it was slapped together to fill an hour they didn't know what to do with, isn't really fair.
It's not a bad episode of Buffy at all. In fact, I think you'd be hard pressed to find an episode this season that does as good a job making use of the entire cast. Even Cordelia feels like an integral part of this story. Even Joyce. Sure, there's some retreading of familiar ground vis a vis Xander's affections for Buffy, but this is used to fuel a great dramatic scene between Xander and Angel in the hospital waiting room. And the premise is kind of cool: a monster that can be seen by sick people, and Buffy making herself sick to fight it is a nice heroic moment for her.
What else am I supposed to say about it, that Sarah Michelle Gellar is really good in it? News flash, Sarah Michelle Gellar is always really good.
By the way, if you want a real example of filler, in which the writers are knowingly padding the season with inconsequential material, look no further than the early part of the third season of Lost, culminating in the dismal "Stranger in a Strange Land." It was this that convinced ABC to set an end date for the show, allowing the writers to better plan and pace the remaining seasons.