Forums used to be (and maybe still) the most standard place online where users discuss.
They are evolving in many ways with sites like Reddit, Q&A systems or simplified versions.
Discourse is also a discussion platform (built with Ruby on Rails) that is not complicated for end-users, eases reaching to the content easy and has various social tools.
First of all, there are no pages to navigate, just scrolling is enough. It is possible to mention users like Twitter and has support for real-time updates.
It remembers where you last left reading, reply to the entries quickly, send a quick-reply-invite to friends so that they can reply without registering and login/register with any social network accounts.
Discourse makes categories optional, they can be created or not, has an admin for controlling everything and an API for expanding it further.