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.