It's the explicit inclusion of period where 'normally' there wouldn't be one. In texting or DMs it would normally be assumed that one-liners wouldn't contain punctuation except to enhance effect, so the inclusion of the full stop is being read as a 😐 or exaggerated neutrality

What about it makes it look passive aggressive?

Good question!

As I explained later in the post, “Great.” looks like sarcasm. My brain interprets it as having a sarcastic tone, and thus being passive aggressive.

(I am not alone in this, hence the very thing we’re commenting on.)

How would excluding punctuation make it not look passive aggressive?

You might as well ask why tone of voice changes the way we interpret things. Written short-form communication has evolved cultural norms that some people understand better than others, just like spoken communication. Chalk my tone interpretation up to an adolescence spent on IRC.

My point is that the full stop being passive aggressive is contextual. None of my uses of it here are intended to portray passive aggression or sarcasm, and if I wanted to do that I would not only change my sentence length and structure, but also my vocabulary.

But of course these norms aren’t as readily understood as actual tone of voice, which is why things like “/s” can be useful.

Wow. What a good question.

The fact that 'great' by itself is not a full sentence, and a period indicates a declarative stop.

See example above.