The moment it became possible for us to destroy all life on earth our first priority as a species had to become... preventing that. A lot of us critters got caught up in insuring our particular nation state survived, rather then trying to save us all.
Securing unsecured nuclear material should be the first priority, but that's just my best guess.
We should also be expanding human life to new places. Space will remain out of reach for a long time, but the ocean floor is quite accessible. There is no reason we couldn't start building there, or trying to adapt human life to an aquatic environment.
Unfortunately, we can't predict what will the be the most pressing issue in the next 20, 50 or 100 years. Over population might be it, but I wouldn't assume so.
Technology has advanced very quickly in the last few decades, and society is still trying to chew all the technology it's swallowed. The answer is not to try to slow the rate of technological advance, that would be impossible, and a handicap to whatever society tried it. As society continues to adapt to existing, and new technologies (birth control for example), we will see the solutions they provide be applied to more and more people. Since we can't predict the rate at which this will occur, for specific technologies and specific problems, it is too difficult to predict which problems will be most important.
Right now we have a greater need for societal adaptation and change, then we have for new technologies. Each society, state, region and group of people needs to reevaluate whether it is having a positive, or negative affect on the other societies of the world.
-Brother Rail Gun of The Short Path
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