That social norm always made sense to me: Simple disagreement doesn't provide much information, unless you provide a reason. Even a short reason gives me a "hook" to evaluate why you might disagree, or to do research.

The exception would be if an expert's intuitive evaluation is saying "this seems wrong to me", at which point I have a good reason to dig in to it myself.

Besides, if you don't want to bother explaining why, then either you're trying to outsource the cognitive cost to me (in which case you probably don't care whether I change my mind), or you don't consider it worth the cognitive cost in the first place - either way, there's no reason for me to believe that your disagreement is worth following up on.

Especially here on LessWrong, the "Vote Down" button seems the simplest way to disagree without explaining any farther, and avoids the social issue entirely.

I think "I agree" is somewhat more acceptable because it at least adds a little emotional bonus of "yay, the tribe supports me!" whereas "I disagree" is a very mild hostile bump of "eek, the tribe might exile me!" That said, I've seen plenty of communities where "I agree" is considered a taboo statement, and I always find it sort of surprising how often I actually see it around here :)

tl;dr: a chorus of "I agree" / "I disagree" is simply adding noise to communication.