I have always thought Weblogs are a simplified version of the Usenet newsgroups. As a matter of fact, my first experience with Weblogs are through a NNTP based aggregator called nntp://rss.
Now that I can compare weblogs and newsgroups within the same interface (granted an interface designed for weblogs, not newsgroups), a few features of newsgroups that I miss the most is clear:
The concept of a thread. The ability to mutate a follow-up into a full-fledged entry.
The delivery of follow-ups the same way originals are delivered.
The ability to have discussions appear to happen in multiple groups, and if I read it through one group, it will be marked read in all groups.
The ability to also email an entry or a follow-up to additional email recipients, at the time of posting.