I'll explain to you the Top 5 things I feel are the most common mistakes people make when they make a brand new forum.
I hope this guide helped you, please don't take offense by it, it's not aimed at anyone in particular it's aimed at those specific people at a whole. If you ever need advice on how to make your forum stand out, you can always ask here and I'm happy to help.
Free domains
The first mistake for me is having a free domain (free hosting comes under this category aswell). Free domains scream "Leave me alone" to users who want to find the forum they're looking for. Who wants to be active on a domain that has the ability to just kick you off it whenever they want? They've been known for doing that as well. Also, it tells the users you're not willing to invest a tiny $5-$10 to get a real domain. If you're not willing to do that, why should they sign up and help you?
Nulled software
Unless you're using paid forum software, this doesn't apply to you. Some people out there are so obsessed with using forum software's like vBulletin and Invision Power Board, that they're willing to download a public leak of the software and upload it to their site, when 99% of them know the risks and don't know how to check for backdoors. Firstly, with any piece of stolen software, you can get sued. vBulletin are known for demanding webmasters to not use their software if they haven't paid for it, and quite right too. Is it seriously worth uploading nulled scripts when they're most probably backdoored, just to use paid software when it's not any better (IMO) than free forum software?
Sections
The good old "let's make lots of sections" error. This is probably the most common mistake new forum makers make. They install their forum and straight away they create 10+ categories with sometimes over 40-50 sections. Why in the hell would you do that? You get tons of sections saying "Last Post: Never" which screams inactivity to guests. Also, you need to remember this quote. "What looks more active, a very large cafe with 10 customers and tons of free seats, or a small cafe with 10 customers with only a couple seats left?". That's one of my favorite quotes out there, because it's so true. When you make a new forum, you only need around 2-4 categories MAX and around 6 sections if that. When marketforums was first created, it had a tiny amount of sections. Once you get more content, activity
and users that's when you expand.
Embarrassing Advertising
You always find that webmaster on another site that PMs you out of no where with "Hey, join my site, it's growing at a rapid pace but we always need new users" when the site is completely empty. That's just unprofessional. 99 times out of a 100 the user who he advertises too will simply not visit it. Don't spend your time PM'ing users and spamming users on another site, that gives you a bad name. Sure, you can advertise within the sites rules, but spend your free time uploading content to your site. Content = Traffic. Google and other famous search engines thrive on content. Don't give your forum a bad name by pissing off lots of people for no reason. Give them a reason to join!
Staff
Giving Staff to everyone who signs up. This is another really really really common mistake. Webmasters give Moderator rights, Staff rights (Global Moderators) and even Administrator rights to people they've never hardly spoken too before. I see webmasters ALL THE TIME make threads saying "sitenamehere NEEDS NEW ADMINS/MODS" and you visit the site and it has absolutely no content whatsoever. Why would a site with 30 posts need 4 new Moderators and an Admin? You should be running the site on your own, and any member who joins only for the promotion you shouldn't even want on your site. Yes, you need to give them a reason to join, but that's your tutorials/niche's job, not promotions. 9 times out of 10 this ends in disaster aswell. You have one argument with them and they can delete your site, what do they have to lose? Ideally, you shouldn't give anyone FTP or full ACP access unless you're really really sure you trust them.
I hope this guide helped you, please don't take offense by it, it's not aimed at anyone in particular it's aimed at those specific people at a whole. If you ever need advice on how to make your forum stand out, you can always ask here and I'm happy to help.