these aren't obvious at all before they happen, not just for first-time startup people but everyone.
Based on the original thought "The best startup ideas are the ones that seem like bad ideas but are good ideas."
That one for sure is culled from a list of outsized successes (because they didn't make sense or had a large amount of barriers to success) but doesn't provide balance in the sense that bad ideas are more typically bad ideas. VC's like to hide behind this type of thinking often but they get around it by making bets on many companies whereas the person starting their own company has more of a reason to worry about the downside of a stupid idea.
I don't think for a second that it makes any sense that "best startup ideas are ones that seem like bad ideas but are good ideas".
Facebook wasn't a bad idea
Linkedin wasn't a bad idea
Amazon wasn't a bad idea (selling books to start)
So how do you define "bad idea"?
Is airbnb a bad idea because "who would want to rent out their apartment to strangers"? Or is it a bad idea because "you will never get regulatory clearance" (ditto for Uber).
Facebook was a bad idea because MySpace had already won.
LinkedIn was a mediocre idea b/c it would be very difficult to get enough user adoption to be valuable.
Amazon was a mediocre idea b/c people want to go to a bookstore and look at a book before they buy it (usually). Also, that business has no moat (anyone could sell books online).
You're right though - ideas that seem bad are more typically bad ideas. I still think Twitter is a bad idea. :-P
Really the corollary to this is that all the obviously good ideas are already being executed by huge corporations with massive resources and its very hard to compete when you are severely outgunned.
Also, it seems like it's important to have an understanding of the domain that is somewhat counterintuitive and contradicts the common understanding that people have about it.
Also, be lucky. Your "understanding" might be insanity.
1) The public, reading about the idea doesn't know the full idea or the future plans.
2) The bad idea can allow a pivot into a good idea, once you have money and resources.
3) Impossible to predict the future and what will happen. Twitter is a good example of this (celebrity mention, mainstream media mention, civil uprisings, etc.)