The biggest mistake you can make when creating a brand new forum is releasing it before it is completely setup, as well as assuming that is it going to be a hit from the very first day you create it.
Some things to keep in mind before starting your very own forums is,
•Do you have the time?
•Do you have the patience?
•Do you have the skill set?
•Do you have the money to invest?
•Do you have a good niche that is unsaturated?
•Have you done the required research on starting a forum?
If you answer yes to all the above you should have no problem starting your own forum, and finding success.
Starting a forum takes so much time its unreal, and once it becomes actually active it takes even more time. Having the patience, and not giving up is key factors as most people that start forums either don't have the patience and eventually just give up. You cannot expect to put a website up overnight fall asleep and it is all the sudden the biggest forum around. It requires daily work that most people can't do.
Also the skills required as very necessary in forum creation as you have to actually know what you are doing, or what do you expect to do when you run into a corner? You can't always rely on someone else to assist you in everything you do.
Money is another major factor in running a forum. You can't just buy a domain, and toss free hosting on it. You actually need a good host that you can rely on. you will also need money to advertise, and get the proper search engine optimization needed in getting traffic to your website. Without money you won't get far. Another good thing to get first from the start is your very own custom and unique theme, one that has been never seen before or thought of for that matter. the more unique your theme is the more people will want to stay and check your forum out.
Your niche is probably the most important factor in a forum. Alot of people start a forum with a saturated niche, meaning there is hundreds if not thousands of other forums on the topic you are wanting your forum to be. If you can find a topic that doesn't even have one forum based on it, you have found a very rare unsaturated niche. It takes alot of time and effort to find a niche that doesn't already have hundreds of forums based off of it. Google adwords keyword tool is very helpful in finding out your competition on your keyword/niche, and just doing a basic search on your niche followed by forum or forums is another great way of finding a unsaturated niche. Considering if you see 10 other forums on the first page you do a search on then clearly the niche already has high competition on the niche and will be hard to start a fresh forum on as its saturated. However if you search and don't see a forum or numerous forums than you have found a niche that gives you more odds of success.
These are just some basic things to think about prior to jumping into creating your very own forum.
Ill add to this from time to time as well.