Getting content and active members, but also i find getting all the rules made and in place around the forum, all the correct stickies with the correct information on them, all the typing and also creating documents, etc.. on your desktop to keep track of things your doing on the forum, etc.. All that can be a task on its own.
Gaining regular content and active members are the hardest part of starting a forum. I'd start by getting regular content on my forum by post exchanges, posting packages, and marketing and then I'd hope a few months of hard work it would pay off and I'd have passive activity. Post exchanges are very time consuming but without any hard work, your community won't be successful. There are other factors but this is the biggest but all the tasks can be done if you put your mind to it.
I believe every answer will be along the lines of activity and attracting members. Managing a forum through upgrades and overcoming technical issues can also be a challenge for those who are new to the game.