Juvia currently also makes no effort to support nested comments. I believe nested comments only make sense for extremely active discussion forums. Instead, Juvia strives for simplicity.
Other notable highlights:
Moderation support, either manually or automatically via Akismet.
Multi-user support. With this I don't mean commenters but users who operate the Juvia admin panel. Each user account is isolated and can only see and manage its own sites, topics and comments, except for administrators who can see and manage everything.
Email notification of new comments.
Wordpress comment import (thanks to JangoSteve).