MetaFilter. It's a very old school general interest indexer/forum. It's been around since 1999, has a lot of well known professionals on it, requires a small fee to make an account, they pay their moderation team a living wage because it is a job, and has recently transitioned to official non-profit organization status.

Quality links, quality discussion, (although discussions are chronological, not threaded or voted on, first post gets top spot) plus some goofy stuff and random quality art thrown in. Kind of general interest, but because of the format, some posts can have absolutely mind-bending numbers of links and in-depth information. Some users go all out in making insanely in depth posts that end up generating significant discussion. As I said, you have a lot of professionals on there, and unlike say, HackerNews, it's professionals of all stripes: scientists, programmers, lawyers, federal workers, economists, writers, musicians and so on.