My greatest fear is that Google and Facebook will co-opt email and charge money for companies to get access. Google has already made some progress with this by selling ads in Gmail that look like email while pushing commercial messages in to other inboxes.
This is bad because it penalizes companies with low margins while shifting the advantage to those who are able to squeeze more money out of the users (you can see the effect of this on Google Search, Facebook, and now Amazon. It isn't pretty.)
A DIY home mailserver will still let, like, 5-15% of spam emails slip through.
The last few times I went to spam and saw emails that shouldn't be in there, a careful inspection revealed they were actually very well crafted phishing messages.
Ever since, I don't bother checking the spam folder. Too dangerous. Gmail is smarter than me.