The greatest dangers, I think, are those that sneak in through the back door when you're not looking, and move so slowly that you can't really see them at all until they strike. Climate change, for example, may turn out to be one of these; the scientific community has been talking about the danger for decades, but it's only been in the last few years that the general public has begun to pick up that awareness.
One potential threat that's still a long way out, but which deserves consideration nonetheless, is the long-term danger of orbital warfare. I touched on this subject back in May, when I criticized the laissez-faire attitude George Friedman displayed toward it in his "The Next 100 Years." I was reminded of the necessity of addressing that threat early by an article in yesterday's Toronto Star, which discussed the possible future viability of space-based solar power.
If you're unfamiliar, space-based solar power is a system in which orbiting solar generating stations gather and wirelessly transmit electricity to Earth-based receivers. The concept has been around for a while - its first great heyday was the early 1970s, when the oil embargo made the prospect of non-fossil power attractive, and gamers of my generation may remember the Microwave Power Plant from 1993's SimCity 2000, which worked in this way but had the failure mode that errant bursts of wireless power might vaporize parts of your city.1 Fortunately, in reality this is about as likely as a nuclear power plant exploding like a nuclear bomb.
One very attractive aspect of space-based solar power is that, once the satellites have been built and launched, they produce zero-carbon electricity. Multiple solar power satellites in orbit would do wonders for a planet struggling to increase or retain its standard of living even as attempts to do that further destabilize the ecosystem. The Star reported that Japan is working on its own "moon shot" initiative of launching a 1,000-megawatt solar power satellite within the next thirty years.
The problem, though, is what happens when they're up there. Satellites are uniquely vulnerable platforms. In a world with no active orbital cleanup, debris will accumulate and the threat of a Kessler Syndrome will remain ever-present. Furthermore, they're vulnerable to attack as well. One kinetic missile could turn a solar power satellite into a million constellations of shrapnel. This is exactly what happened in the historical background of Transhuman Space - the Transpacific Socialist Alliance was highly dependent on solar power satellites for its electrical generation, and in its short war with China effectively all of those installations were destroyed.
The incipient importance of orbit makes the idea of a war fought there ludicrous. Orbital fighting would not have the "cleanliness" of modern warfare - it would resemble more H.G. Wells' nuclear weapons, no more powerful than conventional explosives but which continued exploding for days. If governments become wise to this before it's too late, the best thing in my mind would be for attacks against orbital infrastructure to be treated the same as attacks using weapons of mass destruction.
Because, really, that's exactly what enough debris will do in orbit. It just won't happen all at once. We have our opportunity for a fresh start in space, to improve the whole world with the resources we can find there, but we can't do it recklessly or we won't have it at all.
1 On the other hand, it has the second-lowest cost per megawatt of any power plant in the game, behind only Fusion.