Governments. The harder it is to enforce things like tax collection, economical regulation, online behavior, etc, the more power it will seek. And with AI, robots and so on, that is a scary combination.

Super intelligent AI. IMO there's at least a 30% chance we will create an AI that surpasses the intelligence of humans by 2050. What then?

No need to worry. You'll even feel smart when this superhuman AI takes your advice. You won't even realize what that actually means.

1. Climate change

2. The awful power of a billion online idiots