What type of corners do you prefer I a forum design? I think the squared look matches certain specific niches of forums like gaming, roleplaying, political discussions and the like. The round corners are more suited to niches handling laid back an offbeat topics. Besides, I think that the rounded corners are more appealing, especially when the board index is combined to around 60 - 75% of the screen space available.
Honestly, it all just depends on the forum design you are looking at. Example, I once made two designs for a gaming community. One year it was squared off corners, the next year it was 3 pixels rounded. Then again this year they are looking to hire me again to make a new theme, and I think I am going to go with square ones again So I'm not sure it has anything to do with any specific niche. As you can design anything related to any niche.
I think it just depends on the overall look and feel you are wanting out of the specific forum design.
two of my theme are rounded and my other new one is squared. and indeed it depends on the theme and layout and how well it goes to suit the website. It's also the same as the profile image as I got an cycle shape other then squared.
To be very honest, I like the cornered ones. To go even further, cornered and non-gradient. In the past, rounded gradient boxes had their place... this was a time before mobile devices were really popular and the expectation was a majority of the internet was on desktop systems.
Unfortunately, the scale has tipped, and now a majority of users use mobile devices. Cornered boxes without gradients are much cleaner and more appealing both eye and functionality wise on mobile devices. This is especially true when considering the need for responsive design. There are so many screen sizes out there now (and the ability to make your own size). It is absolutely crucial now that websites are responsive. Responsive elements move around the page as the screen size changes in order to best accommodate the screen size the user is using. Rounded objects would have a harder time moving around appropriately and looking appealing while doing so, especially if the element has to be squished for the screen.
So I'm not sure it has anything to do with any specific niche. As you can design anything related to any niche. 
