I used to think I could tell, but I can't actually guess accurately based on writing style alone. (Topic choice, sure.)
Writing style can't be strictly separated from the choice of topic (or rather sub-topics addressed when writing on a given topic), and some of the most powerful clues come exactly from where these things blend into each other. Moreover, in interactive back-and-forth writing on forums and blogs, typical male and female behaviors and attitudes often quickly become apparent, just like in a live conversation, and are clearly detectable in writing.
Someone has already posted a link to a paper whose authors claim to have found measurable statistical differences between male and female styles, but I'm not sure how much (if at all) the usual human intuition relies on those specific clues.
But in any case, I don't see where exactly you disagree with my above diagnosis, given that it's discussing what I believe to be a fairly extreme and clear-cut case. Do you think Alicorn's style doesn't have the characteristics I described, or that such writing isn't statistically likely to come from men?
If you want, though, we can see if you can guess the gender of the authors of a few writing samples where the choice of topic doesn't give it away.
If you have some samples ready, I'd be curious to give it a try.
Two writing samples, one male and one female, from humanities journals: First:
...Machiavelli's advocacy of force and fraud in the conduct of politics is the key teaching that has secured his reputation as "Machiavellian" and that has led to the conception of "The Prince" as the first document in the Western tradition to lay bare the dark, demonic underside of civic humanism. But this interpretation overlooks the degree to which a politics of intense competition and personal rivalry inhabits the humanist vision from antiquity, producing
Daniel Dennett has advanced the opinion that the evolutionary purpose of the cuteness response in humans is to make us respond positively to babies. This does seem plausible. Babies are pretty cute, after all. It's a tempting explanation.
Here is one of the cutest baby pictures I found on a Google search.
And this is a bunny.
Correct me if I'm wrong, but the bunny is about 75,119 times cuter than the baby.
Now, bunnies are not evolutionarily important for humans to like and want to nurture. In fact, bunnies are edible. By rights, my evolutionary response to the bunny should be "mmm, needs a sprig of rosemary and thirty minutes on a spit". But instead, that bunny - and not the baby or any other baby I've seen - strikes the epicenter of my cuteness response, and being more baby-like along any dimension would not improve the bunny. It would not look better bald. It would not be improved with little round humanlike ears. It would not be more precious with thumbs, easier to love if it had no tail, more adorable if it were enlarged to weigh about seven pounds.
If "awwww" is a response designed to make me love human babies and everything else that makes me go "awwww" is a mere side effect of that engineered reaction, it is drastically misaimed. Other responses for which we have similar evolutionary psychology explanations don't seem badly targeted in this way. If they miss their supposed objects at all, at least it's not in most people. (Furries, for instance, exist, but they're not a common variation on human sexual interest - the most generally applicable superstimuli for sexiness look like at-least-superficially healthy, mature humans with prominent human sexual characteristics.) We've invested enough energy into transforming our food landscape that we can happily eat virtual poison, but that's a departure from the ancestral environment - bunnies? All natural, every whisker.1
It is embarrassingly easy to come up with evolutionary psychology stories to explain little segments of data and have it sound good to a surface understanding of how evolution works. Why are babies cute? They have to be, so we'll take care of them. And then someone with a slightly better cause and effect understanding turns it right-side-up, as Dennett has, and then it sounds really clever. You can have this entire conversation without mentioning bunnies (or kittens or jerboas or any other adorable thing). But by excluding those items from a discussion that is, ostensibly, about cuteness, you do not have a hypothesis that actually fits all of the data - only the data that seems relevant to the answer that presents itself immediately.
Evo-psych explanations are tempting even when they're cheaply wrong, because the knowledge you need to construct ones that sound good to the educated is itself not cheap at all. You have to know lots of stuff about what "motivates" evolutionary changes, reject group selection, understand that the brain is just an organ, dispel the illusion of little XML tags attached to objects in the world calling them "cute" or "pretty" or anything else - but you also have to account for a decent proportion of the facts to not be steering completely left of reality.
Humans are frickin' complicated beasties. It's a hard, hard job to model us in a way that says anything useful without contradicting information we have about ourselves. But that's no excuse for abandoning the task. What causes the cuteness response? Why is that bunny so outrageously adorable? Why are babies, well, pretty cute? I don't know - but I'm pretty sure it's not the cheap reason, because evolution doesn't want me to nurture bunnies. Inasmuch as it wants me to react to bunnies, it wants me to eat them, or at least be motivated to keep them away from my salad fixings.
1It is possible that the bunny depicted is a domestic specimen, but it doesn't look like it to me. In any event, I chose it for being a really great example; there are many decidedly wild animals that are also cuter than cute human babies.