Note: The not ELI5 version is Nick Bostrum's Superintelligence, a lot of what follows derives from my idiosyncratic understanding of Tim Urban's (waitbutwhy) summary of the situation [0]. I think his explanation is much better than mine, but doubtless longer.

There are some humans who are a lot smarter than a lot of other humans. For example, the mathematician Ramanujan could do many complicated infinite sums in his head and instantly factor taxi-cab license plates. von Neumann pioneered many different fields and was considered by many of his already-smart buddies to be the smartest. So we can accept that there are much smarter people.

But are they the SMARTEST possible? Well, probably not. If another person just as smart as von Neumann was born, the additional advancements since his lifetime (the internet, iphones, computer based off of von Neumann's architechture!) can use all of these new inventions to discover even newer things!

Hm, that's interesting. What happens if this hypothetical von Neumann 2.0 begins pioneering a field of genetic engineering techniques and new ways of efficient computation? Then, not only would the next von Neumann get born a lot sooner, but THEY can take advantage of all the new gadgets that 2.0 made. This means that it's possible that being smart can make it easier to be "smarter" in the future.

So you can get smarter right? Big whoop. von Neumann is smarter, but he's not dangerous is he? Well, just because you're smart doesn't mean that you'd be nice. The Unabomber wrote a very complicated and long manifesto before doing bad things. A major terrorist attack in Tokyo was planned by graduates of a fairly prestigious university. Even not counting people who are outright Evil, think of a friend who is super smart but weird. Even if you made him a lot smarter, where he can do anything, would you want him in charge? Maybe not. Maybe he'd spend all day on little boats in bottles. Maybe he'd demand that silicon valley shut down to create awesome pirate riding on dinosaur amusement parks. Point is, Smart != Nice.

We've been talking about people, but really the same points can be applied to AI systems. Except the range of possibilities is even greater for AI systems. Humans are usually about as smart as you and I, nearly everyone can walk, talk and write. AI systems though, can range from being bolted to the ground, to running faster than a human on uneven terrain, can be completely mute to... messing up my really clear orders to find the nearest Costco (Dammit Siri). This also goes for goals. Most people probably want some combination of money/family/things to do/entertainment. AI systems, if they can be said to "want" things would want things like seeing if this is a cat picture or not, beating an opponent at Go or hitting an airplane with a missile.

As hardware and software progresses much faster, we can think of a system which could start off worse than all humans at everything begin to do the von Neumann->von Neumann 2.0 type thing, then become much smarter than the smartest human alive. Being super smart can give it all sorts of advantages. It could be much better at gaining root access to a lot of computers. It could have much better heuristics for solving protein folding problems and get super good at creating vaccines... or bioweapons. Thing is, as a computer, it also gets the advantages of Moore's law, the ability to copy itself and the ability to alter its source code much faster than genetic engineering will. So the "smartest possible computer" could not only be much smarter, much faster than the "smartest possible group of von Neumanns", but also have the advantages of rapid self replication and ready access to important computing infrastructure.

This makes the smartness of the AI into a superpower. But surely beings with superpowers are superheros right? Well, no. Remember, smart != nice.

I mean, take "identifying pictures as cats" as a goal. Imagine that the AI system has a really bad addiction problem to that. What would it do in order to find achieve it? Anything. Take over human factories and turn them into cat picture manufacturing? Sure. Poison the humans who try to stop this from happening? Yeah, they're stopping it from getting its fix. But this all seems so ad hoc why should the AI immediately take over some factories to do that, when it can just bide its time a little bit, kill ALL the humans and be unmolested for all time?

That's the main problem. Future AIs are likely to be much smarter than us and probably much more different than us.

Let me know if there is anything unclear here. If you're interested in a much more rigorous treatment of the topic, I totally recommend buying Superintelligence.

http://www.amazon.com/Superintelligence-Dangers-Strategies-N... (This is a referral link.)

[0] Part 1 of 2 here: http://waitbutwhy.com/2015/01/artificial-intelligence-revolu...

Edit: Fix formatting problems.