What are the biggest leadership challenges?

I think the biggest challenge that leaders can have is related to decision making. Difficult decisions often cause resentment and clashes of interest/goals. Possible attacks and pressure leader has to handle can be toughest one.

If we are talking about leaders of civil society organizations specifically, then they can have problems with politicians who either don't cooperate, completely ignore problems of civil society or their corrupt manners can show up.

Also, problems with self-interest can arouse. Better yet, self-interest of his/her family, friends, former business associates... If leader is weaker he/she can give in the pressure and make decision that will be harmful for him/her and whole society/organization.