There are three key challenges facing entrepreneurs today.
Funding/financing: If you do not have the proper startup funding, you're obviously going to be behind the 8-ball. Can you effectively pitch angel investors or VCs? Do you have enough money saved up personally or the resources to bootstrap? Can you produce a successful crowdfunding campaign? And once you do have the funding, do you know how to manage it? There are a lot of stresses with funding.
Passion: Most entrepreneurs will have the drive and passion they need to succeed early on, but when they face adversity or when they fail the first time, will they have the motivation to keep going? Also, going back to the beginning stages, if this person has a secure job and good salary, would they be willing to drop everything and venture off into the unknown to build their dream business? Your passion and determination should be what motivates you to succeed.
Unknown aspects The majority of startups fail. So what will make yours any different? How long will your business last? Will you be able to attract subscribers to your website or enough customers to your product? How long will it be until you turn a profit? There are so many questions that entrepreneurs constantly need to ask themselves, and constantly need to find solutions to.
Also, things like not doing proper market research or a competitive analysis may destroy your business before it even begins.