What do you think is probably the hardest thing about running a message board/website?
I would think designing is the hardest along with getting members (for a message board at least).
Designing it. That's what everyone sees when they visit and first impressions are the one that always remains with you.
Design's obvious, but if you run a message board getting members to join and dealing with those bad ones can be pretty hard at times.
Besides design, I would think managing the back end stuff would be pretty hard (especially if the site's in PHP) because you have to worry about things breaking and the site getting hacked, etc.
The hardest part of running a website/forum is making it successful and keeping it that way.
Personally I find it's all those little trivial things that make running a site to be the hardest (like fixing broken html, or constant updates or the fact you gotta make sure anything you design works in Internet Explorer).