Here are some differences between these eras: nowadays there are (a) more places to debate online, (b) more people online, and (c) lower quality of an average person online.

The old model probably wouldn't work now. I mean: Imagine that you create a new discussion forum to discuss, say, rationality. This is as easy as spending 2 minutes on Reddit website. Cool, you have a forum. What now? Because, if you can't answer this, then a few years later you will likely have an empty forum, or a few random crazy or bored people, an occassional spambot, and generally nothing you wanted to have.

Two quick solutions: bring initial people, or bring initial content. Eliezer did the latter. I assume you prefer the former... but then you need to know a dozen high-quality people interested in the same project to start it. Which would be rather easy decades ago at a good university, when you were physically surrounded by smart people with similar hobbies, and they didn't have much other options on internet anyway (at least this was my experience). But these days, there are millions of websites, people have different preferences (e.g. some of them prefer to write their thoughts on social networks, others hate social netwroks; some prefer e-mails, some prefer websites; etc.), so it is difficult to organize them. Not impossible, just... unlikely. For example I consider mailing lists horribly inconvenient, and leave them quickly even if the topic is very interesting; the user interfaces I tried (e-mail client, google groups website) feel painful. I know people who react the same way to web debates. So I guess some people are just too spoiled to have a discussion at the same place.

Are there some mailing lists you would recommend as a better alternative for LW? I'd like to see them (specifically something about spreading rationality). Or are you complaining generally, that the whole internet is getting worse and the old-style debates are no longer possible?