He's done it, what, three times? In fourteen seasons (twelve and two-halves technically) of television and one movie? Not exactly an epidemic or an overused storytelling strategy.

For the record, I'm counting Tara (Buffy S6), Fred (Angel S5), and Wash (Serenity). Joyce could be argued into that category, but she was never a main character. I don't think Doyle (Angel S1) counts because his death wasn't a shot to the guts, and he was designed as a placeholder character. Buffy, Angel, and Spike's deaths (S5, S2, and S7 respectively) were reversed and weren't really done in a gut-punch, heart-break way, and Anya's (S7) was just weird. Oh, and Book's wasn't a gut-punch either, nor Wesley's, nor Cordelia's.

The few times he's really, seriously punched his fans in the guts by killing a lovable character seem to have had such an impact that they stick in everyone's minds and stand out in their overall assessment of his storytelling.

He definitely doesn't hesitate to kill his characters, though. If someone wants to criticize him for that, there's really no defense, he does it. But it seems like the only ones people get mad at him about are the three I mentioned, the gut-punches, and those are just strong storytelling.

The ones people should be upset with are the ones that really served very little purpose (Anya, Spike, Cordelia) (I know the last was dictated by circumstances outside of the show).