In small or restricted communities, that can definitely be the case. But I've never seen a large community with good comments that didn't have some or all of the above features.
The one that comes to mind immediately is MetaFilter. Not huge, but a pretty decent-sized community and above-average comments. No threading or non-chronological sorting involved.
John Scalzi's blog Whatever also has a pretty large pool of commenters in a standard Wordpress setup.
The thing both of these have in common is active moderators.
Have you?