There are mailing list clients that will (try to) thread conversations for you. The Google Groups based ones display that way, you can (generally) read them without signing up. And doesn't Gmail attempt to thread conversations? Nothing says you have to use one mailbox for everything ... I use three, a main Fastmail one, a backup Hotmail one that gets one type of traffic that tended to not get through Fastmail or whatever vendor I used before them, and another Hotmail one for a thinly disguised nom de guerre.

Serious newsgroups of the sort you'd be interested in should be covered by http://gmane.org/ , and when I last subscribed to a professional newsgroup service, they and the general netnews community were good about blocking most spam. Look at this one I just picked mostly at random: http://dir.gmane.org/gmane.comp.ai.prolog.swi

Don't know about forums, but presumably there might be some out there. Spend some quality time with gmane and you might find links to some.

Also look for bloggers who have quality comment communities. And of course you'll be able to engage some of them in their comments sections.

Good luck!