I actually gave a TEDx talk on this topic, my conclusion (similar to PGs, but more succinctly summed up in 10 words) was any time you find yourself saying "this is stupid, there has to be a better way..." BINGO! you've got a startup topic. Just actually keep track of it, and you'll find it happens more often than you realize.
btw, Thomas Edison didn't invent the lightbulb. Lightbulbs were around. He improved on it. Actually, his lab assistants did most of the work. And speaking of necessity is the mother of all inventions, Thomas Edison's phonograph invention (again, it was his lab assistants) came before he applied it to a need. It took him a while to figure out to apply it to the music industry.
Thanks for listening. I debated using the TE example beforehand, I am ashamed to say that I gave that speech knowing he wasn't the real inventor. I needed to connect to my audience, this was in another country and TE is one of the few inventors on their knowing.
The talk, should anyone actually want to listen to me talk for 20 minutes: http://youtu.be/bPwgkf1aZ9I
(the guy controlling the projector had a hard time with the remote, the background slides keep going out of order :(