Overpopulation - especially in regards to lack of available or useful resources, less so in terms of space. Though life in places such as Tokyo are undoubtedly crowded and hectic, they show that many other cities around the world could hold a significantly larger population, though this feeds back into the resources problem of providing the materials and infrastructure to house that many more people.
A solution to this is the controversial implementation of a lottery or test system by which Earth's population growth can be slowed so that when inevitable problems come around, it devastates less people and less resources are consumed. Of course not everyone is going to like this, but even if the world doesn't go to hell in a hand-basket, upping the production and efficiency of goods and services is still not going to cut it for a population as large as Earth's will become at its predicted rate. There will be at least a few wars raging at any particular moment, as nations fight to obtain or defend resources. As resources become so stretched, and human life so plentiful, the painful question of the true value of a human life will also come into question.
Fossil fuels will run low, and as the last, least-inaccessible reserves are desperately drilled into, fuel prices will skyrocket, and the sheer cost of owning a car will see many, many more people make use of public transport, carpooling, or simply walking or riding their bikes. When those last reserves dry up, though, something like 99% of Earth's transportation systems are rendered unusable. Needless to say, that will have a crushing effect on basically every economy on Earth.
Extreme weather patterns will become commonplace anywhere not near the equator - not necessarily things just becoming hotter, as many believe incorrectly. For example, the Australian state of Victoria had been in a state of severe drought for a decade, and now we've got massive floods sweeping across large areas of eastern Australia. Some think this is a sign global warming and climate change are false, but it is direct evidence for it: These floods are a once-in-a-century even, and the last time they occurred was definitely less than a hundred years ago. Coastal towns get washed away by rising sea levels, coastal cities become like Venice.
Eventually, when the polar ice caps melt, the dynamic that provides Earth's climate with warm and cold weather will slow down and eventually stop, and with no climatic heat being produced or maintained, things start to get very cold, only adding to the problems plaguing humanity for the years that preceded it.
