Emails have several issues: Large % of emails are newsletters, notifications, offers and promotions. This is not spam (I think we have got that under control).

Emails have several issues: Large % of emails are newsletters, notifications, offers and promotions. This is not spam (I think we have got that under control). These are the things you intentionally subscribed to and "nice to know" but not important or urgent. Email clients are utterly oblivious to identifieng and ranking them (Gmail's Priority Inbox is rather dumb baby steps). Replying to threads and quoting previous replies is a pain. Threads become too long with several different colored highlights all over. There is no easy way to control your membership in email conversation. It's hard to get out and hard to get in. Creating groups is high friction. Sharing previous conversations with someone or a group is non-practical. Most email clients rank emails using date time. You can say that 70% of the human generated content uses perhaps most naive ranking algorithm. It is mind boggling that we still don't use signals like age of conversation, length, participants, topic, embedded action items etc. Even in 2014, most email clients will happily push email sent at 9AM for your house on fire after the benign Groupon promotion sent on 10 AM. Emails are free form and there are poorly defined standards standards to add structured data such as reminders and action items for recipients, auto-expiration, callback number etc. You can't share your "like" or upvote/downvotes for emails sent to a group. This severally limits how much social expression can be attached to emails flowing within a group.