Remove 140 character limit: That is the essence of twitter. The limit helps me consume a lot more tweets than I can consume long winding emails or articles.
Whitelist: So make it difficult for someone to send you an email unless they are pre-approved? That looks like solving the spam problem with too big a hammer. I think the spam problem is mostly solved btw. I don't get it on either my Hotmail, yahoo or gmail addresses and an occasional spa can easily be marked and future emails get filtered
Hashtags: Google tried to do this in gmail as labels and I think failed. The automatic classification that Hotmail started (to identify important email, email from contacts, etc.) and probably is there in gmail too is the direction. Why make me do the wok when a machine can do it for me?