Ah, the 'Southern Secession' timeline. That's a great one! I think it was a generally good TL—it's one of my favourites—though there are a few things that aren't quite perfect.
For the alternate USA, I don't really agree with the criticisms presented here. Anti-slavery doesn't mean anti-racist. Just look at New England; the Yankees weren't exactly eager to admit the Dominicans into the Republic, even though they liked the idea of having more land for themselves. The way the good-USA's own racism prevented it from expanding—when there's not really anything else that could have done—was excellently done. In any case, the good-USA isn't just New England; there are still racists in it, so it makes sense for there to be racist attitudes even though slavery is abolished there.
There is one thing about the good-USA that I did find unrealistic, though, and that was its good relationship with Britain. Manifest destiny might only have been talked about by Jefferson Davis, but the ideas behind it were around since before the foundation of the United States. The USA broke away from Britain in the first place to a great extent because of the Proclamation Line limiting American expansion. That ideology—of opposition to Britain and of the American people's right to expand—is hard to get rid of. I think the way the Second American Revolution ended in Decades of Light wasn't enough of an incentive to prevent the Americans from expanding into Canada, though there was a good reason provided for why they didn't annex Mexico. I don't think there was ever a good reason presented as to why the good-USA didn't conquer Canada: lots of available land right there, and without the strong British power, allied with New England, that prevented such a takeover in OTL. It would have been more sensible for the author to have the good-USA take over Canada in that war, and then they would be able to plausibly have stable relations with Britain.
I'd have to vehemently disagree with the suggestion that Decades of Light Germany is implausible, though. Germany losing the First World War sounded perfectly reasonable, and so did its reaction afterwards; it was a situation very similar to post-North American War New England, so it's entirely sensible that a vitalist-esque regime could emerge. And for vitalist regimes to start wars for no good reason is entirely in character—just look at OTL. It's not as if New England's national interests were threatened by the possibility of Germany winning the Great War; Mullins went in purely for glory, power and ideology (in OTL, New England's long solidarity with Britain), just like the fictional Hitler went in for glory, power and ideology!
Sure, the nastiness of the 'National Socialists' was exaggeratedly cartoonish—the way they deprived people of the races they didn't like of all public status and humiliated them was realistic and Mullins-esque, and the way they confined them to ghettoes was a reasonable extrapolation of that, but the outright industrialised mass-murder was stretching plausibility, there's never been anything like that—but the key point is absolutely right.The author was making the point that "it can't happen here" is an attitude both dangerous and wrong. Germans might like to think of themselves as "inherently more civilised" (or some such nonsense) than Yankees but it simply isn't true. No country is inherently morally superior to any other.
The idea of having socialism actually succeed in taking over a country was a fascinating experiment, and I disagree with the suggestion that it just couldn't have happened. The author went to ridiculously great effort to make Russia ripe for revolution. I have to say, the assassination of Alexander II just at the right time to give people a taste of reform and then cut it off with the rise of Alexander III, and its effect on Nicholas II to make him reactionary too, was a masterstroke in how to shape the legitimate Russian government's behaviour to make it more vulnerable to revolution. But what I disagree with is how it fell.
Having gone to such effort to create this lovely, non-evil USA, the author seemed to grow a bit too fond of it, and wanted to make it win all the time. I mean, sure, the socialist Russia wasn't very nice, but the way it suddenly fell apart didn't make any sense. It's as if the author wrote himself into a corner; he created a bipolar world when he wanted the good-USA to win, so he had to hastily get rid of authoritarian socialist Russia at the end. I mean, how did a man like Gorbachev, who basically dismantled the authoritarian state structure that kept socialist Russia around, possibly get into power in the first place? Surely such a democratic idealist could never have risen so high in a cruel authoritarian regime.
So I think Decades of Light really was a genuinely realistic and really good TL, up until the authoritarian socialist regime in Russia fell apart. I think it would have been a better ending in terms of plausibility—though, I suppose, less thematically appropriate for such a utopian TL—for the Warsaw Pact to still survive, facing off against the good-USA and its Western European allies indefinitely, at the end of the story.