Indeed. While more technical details such as SEO and adverts shouldn't be overlooked after you've been established, the very foundation itself is highly important.
Forums are already something of an archaic breed. Reddit, social media outlets, and live chat communities have overshadowed the once thriving internet message board community. These days, the only boards I see that have any lasting success are company boards (ie, a board created by the developers/publishers of a video game series), tightly-knit community boards (small, but have survived long enough that the community isn't going anywhere), behemoths (enormous boards that capitalized on general interests / markets before the subject became overly saturated, and possess a massive, replenishable memberbase), thinktanks (boards that possess an accessible wealth of knowledge, assets, resources, etc, for whatever subject they cover) and niche boards (boards that appeal to a more narrow interest, one that isn't overly saturated, and/or a board that has found a creative gimmick unique enough to entice membership). There are exceptions, but they're fairly uncommon.
A significant problem I see with newer owners are attempts to establish generalized forums (ie, catch-all gaming forums, general chat forums, sports forums, etc) The broader the interest, the more likely it is to fail. People generally don't go looking for a community forum that covers broad subjects. How often do you search for "general chat forum" or "gaming forum" in Google? Even with superb SEO, the likelihood of your forum popping up in narrower searches is fairly small, and there are much more convenient sources for general interests (aka, Reddit / Discord / Social Media / Popular forums that already cover that topic). Furthermore, it can be very difficult to sustain a general forum in the beginning. Your members will normally only want to discuss topics they hold interest in, and a broader board, even one limited to a specific genre, will still hold a wide variety of potential subjects. Your budding, fledgling community may end up with members that are passionate about subject matter within that genre, but possess no interest in the topics that your other members want to discuss. It often results in 90% of the forums on a board being empty or only sporadically active. This is why a niche and/or creative gimmick is a much safer bet when trying to get a community up and running. A narrower subject will better unite common interests, and interesting gimmicks invite curiosity.
That's not to say generalized boards can't work of course, but it requires a lot of dedication. At the start, I recommend word of mouth more than conventional advertising or SEO. A polite invitation, when appropriate, goes a long way, and individuals that receive personal invitations tend to at least pop in out of curiosity. Invite some friends, if possible. Once you have a sustainable community, then you can worry about proper advertising.
