The behavior of the moderators. are the biggest deterrent to those possibly interested in opening an account, and maybe even later contributing.
I know several people that have signed up, asked a question, and then were pounced upon by the modsmoderators. After two or three times of that, they just bail.
I have started seeing blogs and other comments on the Internet lamenting the behavior of the mods.moderators on SEStack Exchange/SOStack Overflow as well.
A friend of mine started using SOStack Overflow as he began to teach himself PythonPython. He made the mistake of responding to the mods'moderators' questions thinking they had his best interest in mind. Boy was he wrong about that. He was more and more targeted each time he asked a question.
Then he tried to to contribute by answering a question, and then the very next question he asked was attacked so fiercely it was almost funny. One mod.moderator marked it as a duplicate, even though the supposed original question had nothing to do with my friend's question. Then my friend made the mistake of pointing that fact out. The next time he tried to ask a question, he found a warning that he was "In danger of being banned from asking questions."
He logged out, and has never logged back in again.
Just as he had develop enough knowledge, and felt an obligation, to contribute himself, he was prevented from doing so by self-righteous and self-serving modsmoderators. A real shame.
People see the nastiness when using the site, and they discuss it with each other as well, and the result is to turn many away.