There's a class of ideas that founders tend to gravitate towards. These ideas pass the basic criteria for idea generation – they are usually interesting (and seemingly doable) to a wide variety of potential founders, users claim that they want this feature or app (via user interviews or other feedback), and there's something emotional about it.

Dalton Caldwell and Michael Seibel at Y Combinator call these ideas "Tarpit Ideas" (a much better term, much better than my "Flying-Car Syndrome"). There's probably no other institution that has seen more nascent startup pitches than Y Combinator, so they are in a unique position to think about this. Some notes on some of the ideas they touch upon in their conversation, "Avoid These Tempting Startup Ideas."

Those were the ideas that they listed in the conversation, but I'm sure there are many more. Some of my own ideas:

It's not that these ideas are inherently wrong or bad, but they require an extra layer of thinking for the questions: why now? why this team?