re: your point about requiring a login to comment. Isn't that good for community building? If MetaFilter let just anyone leave a comment, that would kill it off pretty quickly. Having some barrier to entry can add value, you just need to find the appropriate one for your community.
It depends on the context people are interacting with the site.
A blog is "owned" by the author, and comments are from guests who aren't invested enough in the site to register, but who still make up the core of the community.
On a forum, or Metafilter, or Hacker News, the content is produced and owned by every participant who will likely interact over and over and in the process build up their localized reputation and profile on the site as part of their ownership. I think that any of these could be done effectively without registration, but it's not as common.
(I think partly because there are business reasons for registration -- it makes it easier to opt-in people. Even people without the business reasons follow the convention.)
It does build community, as long as anonymous comments go into a moderatation queue. Otherwise it builds noise. The point of logging in (and having karma) should be skipping the queue.
re: your point about requiring a login to comment. Isn't that good for community building? If MetaFilter let just anyone leave a comment, that would kill it off pretty quickly. Having some barrier to entry can add value, you just need to find the appropriate one for your community.