I'd call Reddit and the Threadiverse and Usenet and such forums. They're just broad, with many different categories, or "meta-forums", as compared to a site with a dedicated-to-a-single-topic forum.
Some other drawbacks of having many independent forums:
You have to create and maintain a ton of accounts.
Different, incompatible markup syntax.
Often missing features (e.g. Markdown has tables; few forums let one create tables)
Some forum systems ordered comments by time rather than parent comment, which was awful to browse.
Often insane requirements to get an account. I can think of a few forums that were very difficult to get access to, either because the "new user" system was incompatible with some email system or just had other problems.
I mean, there are a lot of websites with "comment" sections, which is kind of a lightweight forum attached to a webpage, and they're almost invariably awful.