Diaspora introduced some nice group-like features called aspects. Suppose I would like to chatter with my wife and with my girlfriend on Diaspora (purely hypothetical :) ). If I put them into different aspects, they will never know about each other until some traitor emerges. People I put in one aspect don’t know about people I put into another aspect, posts are totally separate too (I can also post globally.)
In Google Buzz I can also post to a private group, but all people I follow and all people following me are publicly visible for everybody.
In Diaspora, even people who I put into one aspect don't know about aspect itself and don’t know about other people there, unless they commented on one of my posts to that aspect.
Next to Kevin Kleinman and xoen in that Get Satisfaction thread, I think that I should be given an option to allow people which I put into the same aspect to discover each other.
Or, as Andrew Famiglietti said there, maybe I should be given an option to introduce people in my aspect to each other.
I think that rather be a default option, because as soon as a person in my aspect comment on my post, this person will be visible to all other people in that aspect anyway. We're not going to hide people's names on their comments from other members of an aspect, are we?
(It'll be nice to have some graphical representation (graph) showing which people are connected and what they can see. To use HTML5 Canvas?)
Discoverability is a critically important feature for social network to grow naturally. We discussed it thoroughly on Buzz, which still doesn't have enough discoverability.
One way to improve discoverability of people is to have an automated suggestions, based on common interests (hash, keywords in aspect names and/or in posts), on a number of people who follow certain person. Twitter excels in doing that.
Another way of improving discoverability is a good search - discoverability of topics.
Yet another way seems to be a Wall (Front page) which displays most popular (with most comments, with most "likes") posts. Chris Land is constantly advocating for such a feature on Buzz.
Finally, Buzz and, I believe, Facebook have one important advantage over Diaspora: I can post publicly and such a post will have its own permanent URL and will be searchable by Google. I believe for Diaspora to grow it should include similar functionality.
Update: I was wrong. Diaspora does allow public posts with permanent URLs visible for non-members. Interestingly, it looks like comments made to public posts are not visible for outsiders. Look at this post I made on Diaspora: https://joindiaspora.com/p/108052. It is visible for people with no Diaspora account, but there are no comments. However corresponding internal post has a comment: https://joindiaspora.com/status_messages/108052.