Wells Beat Craters
Picture demand as a hole you're digging. A good startup idea is a well: narrow, but deep. A small group of people want it desperately. A bad made-up idea is a shallow crater: lots of people vaguely like the sound of it, nobody needs it.
Microsoft started as a well. Altair Basic had a market of maybe a couple thousand hobbyists, but those hobbyists needed it badly. Google looks like a crater now, but nearly all giant companies started as wells and expanded.
The filter question: who wants this so much that they'll use it even when it's a crappy version one made by a two-person startup? If you can't name them, you have a crater.
“Who wants this so much that they'll use it even when it's a crappy version one made by a two-person startup?”
Key idea
Dig a well, not a crater. A small group that needs you desperately beats a huge group that thinks you're mildly neat.