On the one hand, the attacks on power plants that you allude to are possible. Utilities have been networked and electronically controlled since the 1970s. Nobody builds networks on telephony or X.25 anymore; it's all IP. IP connectivity to insanely sensitive systems leaks routinely; moreover, application-level data sharing between Internet-connected systems and supposedly air-gapped backend systems is extremely common.
On the other hand, the "less serious" attacks you allude to are very very bad. Google and Hotmail aren't national utilities. But they are attacked by state actors because dissident organizations use them to communicate. For that matter, the Internet backbone is a collection of computers sharing information using a decades-old routing protocol for which policy is controlled by regular expressions.
Finally, if you run a startup and happen to say something I disagree with, such as "I think CISPA is a power grab by the content industry", I could today very easily push you off the Internet with a trivial DDoS attack. The people who extorted online casinos with DDoS botnets were not rocket surgeons. When I attack you for disagreeing me online, and you call your ISP, guess what you're going to hear? "You're on your own". It is always very weird for me to see people on Hacker News, a hub for online startup news, downplaying the severity of DOS attacks. I've spent a decent chunk of my career in DOS mitigation and it is not remotely a solved problem.
