As people have said, it helps if you are filling a niche that nobody else has thought to fill. Which is harder and harder as time goes on.
There is very little point in starting a forum on a topic which is already the focus of other forums.
When we started, AVSForum in America was already huge. But we served people in the UK, so we had something unique.
There isn't at the moment, for example, a decent UK swimming pool community. That's no doubt because very few people in the UK have swimming pools. But it means that anyone looking for UK swimming pool advice have to ask at foreign websites.
Agree that you should choose a topic about which you are knowledgeable and have a passion.
If people come to your site for help, they need to know they will get it. Which, until you are enough members to answer the questions for you, means you need to do your best to help people yourself.
Also write articles which will act as a useful resource for people interested in your chosen topic.
If you can recruit some enthusiastic, knowledgeable volunteers to help you, then that would be a good asset.
At the same time if there is no scope for advertising on the topic you choose then you'll struggle to fund your site as it grows. The cost of your wages, servers, organising merchandise, member meet-ups and maybe freelance writers can make the cost of your site build, and you will need to fund what will become a business.
If your topic is, say, train spotting, then there is not much scope for advertising. Notebooks. Maps. Macs.
Whereas with us, we have everything which comes under the umberella of home consumer electronics and home entertainment, so there is miles of scope for advertisers.
Always make your site welcoming to new people. Discourage cliquey behaviour, swearing and ensure members are always respectful.
I disagree to an extent about copying other websites. I think it helps to cherry pick the best elements of other websites. But that applies more to non-forum elements since, aside from a unique design, the functionality of the Xenforo forum will be pretty standard. And that's not a bad thing. Just like McDonalds decorate all their restaurants the same so they feel familiar to customers whichever city they are in, retaining some element of the Xenforo feel will make people feel confident about using your forum. This will apply more as Xenforo continues to earn more market share from vBulletin.
Try to partner with an organisation who have a resource but no corresponding community. An existing website, maybe, or even a magazine if you can find one without a website community already.
Knowing PHP, MySQL, CSS and Javascript helps.
Work hard. I spent all of my spare time and had no holidays for four years before AVForums attained 'critical mass'.