Here's a good article from Neil Patel (co-founder of KISSMetrics). He interviewed a few successful entrepreneurs that had built businesses worth at least $50 million to learn what some of their biggest mistakes were.
By far, the reply that kept coming up is hiring the wrong people. It seems that just about everything else falls secondary to "getting the right people on the bus", as Jim Collins would say.
I agree with the general consensus. Your ideas will change, your direction will change, and the skills you need will change. What is critical is that you can find smart, hungry, driven, flexible people with raw talent. Just about everything else can be learned over time. However, just a couple wrong hires can kill a company's culture and make it extremely difficult to grow because you end up spending more time fixing internal staff problems and less time solving the customers' problems.
We've developed a pretty good system of weeding through candidates effectively. We have seperate interviews where a team of two interviewers' job is to specifically focus on one of three aspects: skills, work/school history, and culture fit. By spending a lot of time on each one, the interview teams can get a really good idea of the candidate, and we're not asking the same question to the candidate over and over.
If you've made a bad hire, believe me – your team knows it. Once you become convinced the person is the wrong hire, it's critical to cut bait early. You are doing yourself, your company, and the new hire a disservice by keeping them around. Let them go find something more suitable for their comfort level. If you don't cut bait, I believe it is hard for the new hire to overcome the psychological downward spiral of 'losers slime'. This is where the internal team is not impressed with the hire and thus becomes just a bit less likely to spend the extra time with them. This, in turn, creates an uphill battle for the hire to get up to speed, and eventually everyone ends up frustrated.