What happened in May 2007?

Of course not every person registering an account does so with good intentions. However we are very good at dealing with disruptive contributions from people we don't know, and very bad at dealing with constructive ones. How many people who were trying to help had their edits reverted with either no explanation or an impersonal (possibly incomprehensible) one? How many of them felt welcomed an encouraged to contribute again?

I should have been clearer - not all SEO-type contributions were or are disruptive. It's just that they weren't interested in 'volunteering' their time once nofollow went into effect. We're talking about a very specific point in time when new editors suddenly dropped off. As I said, I'm agreeing with what has already been said, but I find it 'required but not sufficient'. I was editing during that time period, and I don't remember the bullying of newbies suddenly changing, or some 'Eternal September' effect suddenly showing up. What I do remember is the switch from listing all the sources used in a group under 'Sources' to requiring inline citations. Which was a bit confusing and annoying (especially for articles using only one source), but I honestly don't recall any general revolt against it. Most of the visible effect - still is - is that some editors are too impatient to go back and forth between the source and article, and just create bare url links for someone else to fix later (or fill out everything but the url 'from memory' which creates some odd titles!) But that's okay - I've fixed plenty of those over the years and never felt the need to 'admonish' the original editor. It's certainly possible the inline citations turned off new editors, but I would guess it's because they clicked 'edit' and saw so many citations they couldn't find the actual written material, or thought the material itself had to be formatted in some mysterious way. 'Wikifying' a term is easy enough to figure out, simply by seeing a blue link, clicking on 'edit' and seeing that term has brackets around it, and figuring it out. But that's only 4 added characters - citations take up a lot more room, and those using the cite templates require a lot of memorization. And guess what? A lot of non-techies don't immediately recognize the difference between 'curly brackets' and parentheses, use parentheses, and then can't figure out why it doesn't work. Look - all these things would show up in focus groups, or just by watching a non-techie friend try to edit something. Once someone learns to edit, they forget the details of how they learned, what was annoying, what was difficult, etc.