What does everybody think of the introduction / hello threads that are often started in the general discussion area? Are they productive, do legitimate posters actually use them?

Personally, I find them fairly useless. If you want to post, just post. No need to come in doing cartwheels and leaving a one sentence "Hi everyone!" type post. On top of that, they are a gold mine for spammers. All new forum members need to have to their first post approved by a moderator. It's very easy to automatically enter these benign looking posts for easy approval, thus granting freedom to the rest of the forum.