you might need to clarify the question a bit more, because there are two different goals with bitcoin. one is to make the price go higher, and the other is to get as many people to adopt it.
Either goal is fine if that goal is what you think has the biggest challenge.
For instance, if you think the primary goal of Bitcoin is a high coin value, perhaps you think its biggest challenge lies with security breaches at the exchanges that cause the price to crash. If you think the primary goal is user adoption, perhaps you think its biggest challenge is getting more big name companies to accept it as a currency.
Whatever you think its goals and challenges are is fine for an answer.
