I am not as familiar with the structure of musicals as that fan appeared to be. I couldn't tell you exactly what tropes and emotions are required for a musical to be a "tragedy". But I know, when people discuss stories that are tragedies, what they are talking about. And I am increasingly convinced OMWF does not qualify.
I have said, in the past, something similar to that fan: I've said that the cheerful singing and humorous dancing of OMWF cover up the fact that it's one of the darkest episodes of the series. (I might have named it the most depressing episode of the show, but the episode after it wins that prize by a landslide.) Unlike the author of the meta, I've considered this a strength. The irony of the episode is evident and deliberate - no more so than in Buffy's final solo number, when she bitterly repeats a series of heartwarming clichés. "Every day's a gift", indeed.
But as dark as OMWF undeniable is, is it a tragedy? For me, the basic requirement of a "tragedy" is that a story have an unhappy ending.
OMWF ends with Buffy kissing Spike. Is that an unhappy ending?
There has been much discussion in parts of Spuffy fandom of Spike's metaphorical role as death. Sometimes even literal death: since Buffy did not sleep with Spike until she knew he could hurt her, perhaps Buffy was hoping he would kill her one day while they were having sex. Certainly, I think that Buffy went to Spike out of a self-destructive desire to escape her life or even life in general. In many ways Spike did represent death to her, and was a metaphor for death on the show. But it's more complicated than that.
Buffy doesn't have sex with Spike prior to finding out he can hurt her, but she kisses him twice before that. The first time she kisses Spike (ignoring Something Blue) is in OMWF...right after her stops her from dancing to death and gives her a reason to live that she can believe in. "Life is just this, it's living." Buffy didn't go to Spike for that kiss because she hoped he might kill her. She went to him because she thought perhaps he could keep her alive.
When I first watched OMWF, I thought that Spike's lyrics about life in Something to Sing About were too...well, mature. This is the guy who at the end of the season is insisting the great love is passionate and dangerous and not for old marrieds. But the lyrics have grown on me. Spike has been around for long time, after all, as he points out in Touched. I like the idea that he has that kind of perspective in him, even if it is something that might only be pulled out by a dancing demon.
A tragedy is a story with an unhappy ending, but a mid-season TV show episode is not the end ("Where do we go from here?", indeed). At the end of OMWF, the characters have found the story a letdown: They "kind of won", and their embarrassing secrets are out. And, like life, it's not over. Just living, per Spike's advice, can be a letdown sometimes.
And of course: "The curtains close on a kiss, God knows". Whatever hell Buffy and Spike would bring down on each other, wherever they would go in the future, whatever they would inspire each other to become...that was nice kiss. Didn't look like a tragedy to me.