The conversation always seems to be focused on either finding co-founders or funding. That's really the surface level problem. Essentially the outcome. What's holding you back as a founder? It'd be awesome to dig deeper than surface.
If you are trying to bootstrap your startup, then one of your biggest challenges is trying to find co-founders who can/will put as much skin in the game as you do. Everyone wants you to take all the risks while sharing all the rewards.
For me it is not ideas or building, it is staying focused on what actually matters. It is easy to keep adding features instead of solving one real problem well.
Getting in front and hands of users and making them use your product is the key challenge.
"Why would user choose your product over existing established competitors?"