The thing that kills startups at some level, is the founders giving up.
You need unstoppable people. You want people that are just going to get it done.
One of the keys to focus, and why I said cofounders that aren't friends really struggle, is that you can't be focused without good communication.
A single mediocre hire in the first five will often in fact kill a startup.
Everyone starting a startup for the first time is scared, and everyone feels like a bit of an imposter.
You want an idea that not many other people are working on, and it's okay if it doesn't sound big at first.