He's known for it because the times he's done it are pretty memorable. I might argue that it's the killing of characters in general that he goes back to, not just love interests, but it's not as if it's the only storytelling tool in his box, and it's not as if he does it every time he has a chance to.

(spoilers for Joss shows)

In Buffy he killed Tara, and earlier he killed Buffy herself, and before that a secondary character in Joyce. He was planning to kill Xander near the end of Season 7, but decided against it. He killed Anya at the very end, but as it was the end of the show I don't think it really counts. A lot of shows would kill someone off if they knew it was the very end. Spike "died" but didn't really, and it was also at the very end. The same thing with Angel in season two, it wasn't really a death. He didn't kill off Oz or Riley when he wrote them out of the show, even though he could have.

For me, the signature Joss deaths in Buffy were Joyce and Tara.

In Angel, Doyle's death was planned from the start as part of the story. He killed Connor to get him out, and Cordelia was killed because her actress screwed up the plans for season four by getting pregnant. Fred's death is the only one that stands out as a signature Joss death for me, and she did get to come back as a different version of her character. Wes died at the end, but again, that's the very end. It seems to me that by the end, the show had a feel of a "guys make a last stand" sort of show, and that's why Fred was replaced with a less human, less feminine version of herself, eliminating the last bastion of femininity on the show before the endgame.

Book and Wash were killed in Serenity. Wash is the signature death. At least he resisted the temptation to kill everyone. I was afraid he was going to do it, after Wash's death.

Penny in Dr. Horrible is a signature death, but the whole series was only three episodes long so fans never had a chance to be as attached to her as to the other signature killees.

No one died in his X-Men work, nor (as far as I know) in his Buffy Season 8 comic series. In the movie and tv scripts he writes for others, of course he doesn't have complete control over the characters, so a lack of deaths there can't be counted in his favour.

Any deaths in Dollhouse, as far as I remember, happened at the end when the show was definitely cancelled, so they aren't signature moves.

So five signature deaths in five shows and nothing outside of those shows. I think everyone deserves at least one emotional, unreversable major character death per show. But it's defintely true that he's known for character deaths. I just don't think it's quite one-trick-pony material, because he doesn't fall back on it every time. But it's close, and it's a fair point, because you can pretty much guarantee that someone is going to die in a Joss show. But I think that most (not all) of his deaths have valid story-effect-driven reasons for happening.

My that was a lot of words. I guess I can really yak about Joss shows when I'm allowed to.