Can it make money?
You can't always tell at the beginning, but you can often get a good idea. If you're creating something people want there will usually be a way to make money on it. Things that don't make money cost money and are unsustainable.
Does it forward some sort of ideological goal I care about?
Is it moving the world in the direction I want it to go? Is it making the world a better place according to my own value set?
Is it interesting to me?
If I'm not interested in something, I'm not going to do a very good job at it.
Is it something I can execute on?
I need to be able to at least get to a minimal viable product with resources on. I also need to have or be able to acquire the domain knowledge to at least be competent enough to find the right people to work with me within a reasonable amount of time.