Distribution. You might have best idea and best product. But if no one knows it, then its nothing.
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?"
Distribution is definitely challenging even for startups that are well funded. Funding or no funding, while the latter may help it doesn't change user adoption rate. So many great products unfortunately don't get a chance because of distribution.