Friendships - Users can list each other as friends, and display their friend lists on their profiles. Users can also give their friends special privileges, like access to their private blogs.
Private Messages - Users can send each other private messages, which they can read from their message inbox.
Friends Page - Users blogs may include a "friends' recent entries" page, which displays the recent blog entries posted by that user's friends.
Browse Users - Users can use the "browse" page to browse all the users on your blog network. This can be filtered by information such as location, age, gender, interest, etc.
Interest Keywords - If you wish, you can have keywords on your users' profile pages linked to browse results filtered by those keywords.
Search Users - Users can search for other users based on their profile information. You, the admin, have control over which profile fields users can search with.
Extendibility - BlogHoster's database structure is simple and mostly self-explainatory, so with some basic PHP knowledge you can create scripts that display the most recent user signups, entries, comments, etc., and display them on other parts of your website.
Mailing Lists - If enabled, users can create mailing lists for their blogs. This is a list of email addresses that are notified each time a new entry is posted.
Avatars - Users can upload small photos of themselves, which appear on the blog, profile, and next to their username in search/browse results.

Photo Albums - If enabled, each user can upload photos to their own photo album, which is linked from their blogs and their profiles. Photos can be given titles and descriptions, and thumbnails are automatically generated for easy browsing.
Podcasts - If enabled, users can upload MP3 or other audio files to a special folder in their file manager. They can easily podcast these files by clicking a single link and writing a blog entry.
Other Files - If enabled, users can create other file folders to upload and store files with other formats. You, the admin, can decide what formats are allowed and how much space your users will get.