1) What domain knowledge do you have and what do you like to do?
2) What's changing in the world and in the markets.
I'm convinced that personal knowledge of what's lacking in a particular business domain is invaluable. If you have that knowledge you know what people want and that trumps everything else.
If you don't know anything outside technology itself you could read the paper and look which sectors are in turmoil. I think the best sectors are the ones that are most mature and the ones that are least mature and growing fast.
The reason why the most mature are interesting is that the competition is weak. The competition there are the slowest most bureaucratic behemoths you can imagine. Look at why Apple or Google (or Skype to some degree) could so easily topple the powers of the media and telecoms industries. These industries are very mature, very slow to react and very convinced of their old broken ways. Or look at what happened in the airline industry. That's what you want.
[Edit] Just one example that I have come across recently. I don't know what it's like in the US, but here in europe the postal services are broken. We used to have state monopolies that were slow and didn't innovate but package delivery worked.
Now, the sender often contracts private delivery companies like DHL and they optimise exclusively to the sender's benefit (that's their customer after all). I received a computer from Dell recently. I was told to wait at home from 9 AM to 6 PM. That's outrageous. If I missed the delivery they would try again on the next business day and if that didn't work the package would go back to the sender. They knock, wait for 30 seconds and then they're off. If you're in the bathroom, that's your fault. You cannot collect the package anywhere. That's a broken system that needs a new business model and new logistics ideas.