There is a fine difference, but a difference nonetheless, between "that's a dumb idea" and "you're dumb". The first, to my mind, is just about OK; it's the idea that's being attacked. As I tell people at work - smart people can make dumb statements, it doesn't make them dumb. "You're dumb" is ad hominem and probably just over the other side of the line. People, in my general life experience, conflate the two statement.
On every single one of them, you pretty quickly learn about the hundreds of logical fallacies such as Argumentum ad Ignorantiam, Strawman Arguments and Texas Sharpshooter Fallacies. This understanding is somewhat lacking among the members here.
In the case of those two mentioned by Simon above, if you post the first, its perfectly acceptable because you are "attacking the argument"; a legitimate part of debate. However, other members will expect you to justify why you think its a dumb idea. The second is an Ad Hominem, you are "attacking the person", not the argument. Its not acceptable even with an explanation.
Some might see little difference between them. Frankly, if they cannot understand the difference between being told they are stupid, and being told that their idea is stupid, then that is something they are going to have to learn.
Ill finish by saying this. I am usually direct and blunt in what I post, no tiptoeing around. Its WYSIWYG, others will always know exactly what my position or view is and where they stand with me. I like it that way because it leaves less room for speculation and misunderstanding. Now, anyone who doesn't like my opinion, my views, my directness or what I post doesn't have to read me. I'll give them the same advice I give to people who complain about certain programmes on the television... it comes equipped with a mute button, a channel change button and an off button... you choose which one you want to use. If you just don't read me, this will be of mutual benefit; you won't have to take umbrage at what I post, and I won't have to waste my time talking to you..... simples!
ETA: Just a thought. I would object if a poster told another poster that they had asked a "stupid question", even though its not a direct attack on the person, I believe there is no such thing as a stupid question... questions are the first steps on the path to knowledge and understanding.