1. An imageboard needs to have a reason to exist. If people can get the same or better experience from a more well-known site then no one will go to yours.
2. On the opposite extreme, a board can't be excessively niche or else they will quickly die off as soon as the initial content runs dry, if they can even get off the ground in the first place. If the rules clamp down too hard on what topics can be discussed on a site then that stymies potential activity and makes it hard for a community to develop.
3. People need a reason to keep coming back. That means either an active community to interact with, or for there to be new things to discuss. A board about a weekly airing series will have people returning each week even if it is dead in between, whereas vaguer discuss-anything boards are dead for good as soon as they go inactive since they require constant discussions to keep people engaged (if every active user has visited the site since the last post was made and hasn't added anything new, then that's the end of it).
4. There needs to be a path to get new users to the site. Even dedicated posters slowly peel off from the community and if there is no one to replace them you'll quickly get into a downwards spiral. Having a strong community identity is important to keep people engaged but if that is the -only- appeal of your board then it won't last for long as no one new will join. There also needs to be a way for potential users to find your board (and preferably without spamming other sites).
5. Excessive fragmentation is a killer. If discussion is split over too many boards then none of them, or at best one or two, will have any consistent activity. That turns away new users who will struggle to find active discussions and regular users will just congregate in the few active spaces without ever even looking at the rest of the site. Personally I think an imageboard (other platforms can be different in this regard) should have no more boards than it does (semi-)separate communities, with certain exceptions such as to prevent off-topic discussion from clogging up an otherwise niche topic site. If a board can't maintain its own identity then it only serves to weaken the identity of the site as a whole.
6. Active moderation that is engaged with the community. Imageboards are a major target for spam and if it isn't cleaned up quickly then that's a good way to kill a site. Also if the moderators are excessively 'hidPost too long. Click here to view the full text.