I can see a few:
1. It takes calendar time to get some traction.
This is very fuzzy and hard to measure but it happened to is in 1995. Took us several years to get to a nice income. That may be related to the next point.
2. You may need to try lots of different marketing channels to determine what works.
(I’m planning to read Traction, which is supposed to cover lots of different channels. You evaluate each one.
3. you may need to have a product to evaluate whether ppl will buy.
*Corollary to 2 above: *
Need ACTUAL product to ready each channel (I.e, you need to SELL to truly evaluate each channel.)
4.Don’t know how to reach potential customers to validate the idea.
5. If you build it, they will come.
Once a user tries it, they’ll LOVE it. And there is no way to explain it without building it.