For groups, think of LinkedIn. Where you can have associations, topics, companies, etc. Users posts then show up on their links and you can have group-only conversations and posts. For each group there is an owner or proxy who 'approves' new members.

More Adds:

User Profiles and Badges

1. Number of people in the group and a listing of members
2. User geography/location (can be tied to metrics)
3. User language (preferred)

I like your 'postage' idea for spam and advertising! How about this:

Advertising

1. Option (opt-in for Steemians) for advertisers to pay for viewers of ads (ex .001 SBD to viewers)
2. Option (opt-in for Steemians) for advertisers to pay for upvotes for ads (ex. .10 SBD to upvoters)
3. Mechanism for authors to include ads (1 fixed bar at bottom, or something akin) in their posts and advertisers to pay authors

Metrics

1. Number of post views
   2 Graphic showing number of views over time