I think AIDS may be the most acute problem today. AIDS will be rather insignificant against the problems that global warming and over-population can produce in the long run though.
As well, a huge problem will be the ecological impact of the rise of middle-class populations all over the world. While this is a wonderful thing in that more and more people will live much more comfortable and fulfilling lives, there is also another side to that coin. This could create an extraordinary drain on resources as well as pollution problem unless we start implementing solutions now!
Onward and Upward!