Imagine a substantive Less Wrong comment. It's insightful, polite, easy to understand, and otherwise good. Ideally, you upvote this comment. Now imagine the same comment, only with "obviously" in front. This shouldn't change much, but it does. This word seems to change the comment in multifarious bad ways that I'd rather not try to list.

Uncharitably, I might reduce this whole phenomenon to an example of a mind projection fallacy.

I have a different explanation: this is a status defense mechanism. If you say something that other people find obvious, in a way that suggests that you didn't find it obvious, you lose status-points for not being as smart as them. By adding the word "obviously", you in effect say "please do not infer that I think this remark is a great discovery of mine (and thus that I am ignorant relative to you) from the mere fact that I think it needs to be stated explicitly".

As an added benefit, if the remark turns out not to be obvious to your audience, yet demonstrably true, you gain status for having been smarter than them.

You might think, then, that there is no downside to simply prefacing every statement you think is true with "obviously". Obviously, however ( :-) ), you have to avoid making it transparent what you're doing, and thus restrict your usage of "obvious" to particularly plausible cases. Calibrating this sense of plausibility with your own epistemic powers is one of many mysterious (in the sense of not being spoken about or taught explicitly) techniques of human status negotiation. (And heaven help you if you label "obvious" something that is false...)