I've been using SP for over a month now. A few features you didn't mention:

1. you can type the name of a user directly in the search bar
   
2. you can save 'users' (different from following)
3. you can save 'topics' (different from bookmarking)
4. bookmarks can be tagged
5. There is a new version about to be released, so this feature list is only going to expand and require even more updating lol