What you choose to work on is probably the most important decision in your life. The people you surround yourself with and where you choose to live are also very important because they directly impact what you work on.
Throughout my life I have been given many factors for deciding what to work on. Some of them have slowed down my growth and others have accelerated it.
This analysis comes from the perspective of someone who is in the learning stage of life, compared to the doing stage. It also comes from the belief that you want to do great work. Otherwise (and it's not mutually exclusive), pick whatever makes you most fulfilled.
Skills
Picking projects to develop skills is good, but they should be quick. Don't devote too much time to these because they aren't inherently difficult. When you apply these skills in unexpected areas and ways, that's when you've hit the jackpot.
Impact
Impact is very deceptive. So many projects and ideas never get off the ground because we question "is this important enough". When you're young, don't worry about impact. I'm still figuring out when is the right time to start caring.
As you continue to build stuff regardless of importance, you will start to converge on "important" ideas.
Excited Curiosity
I think this is the jackpot. Pursuing what you're excited and curious about is the easiest way to get obsessed about something, which increases the chances you get to the edge of a field. Once you're there, you can push the boundaries of what people think is possible.
This idea is heavily influenced by Paul Graham's How to do Great Work.
