I've been told that one way to come up with ideas for startups is to put a timer on your watch/phone that goes off every 30 minutes. When that alarm goes off, stop what you are doing and ask yourself "Why does this suck, and how could I fix it?". Keep a journal of the ideas and see what you come up with over time.

I think that if you are a 25 year old guy living in San Francisco/Chicago/NYC/insert your favorite major US city it is really likely that you are going to come with ideas that solve a common set of problems. Are those business worth building? Maybe so. Are they going to make an impact on a global scale? Maybe not.

Perhaps if you combined an entrepreneurial spirit with travel to emerging/developing nations and put yourself in uncomfortable or unusual situations you could come up with something that had the potential to change the world.