WordPress.com is a multiuser blogging platform and free host. It's not a wiki host or a forum host. You can get either a wiki or a forum on the internet and link to it form a WordPress.com blog but you cannot embed a wiki or a forum into the blog.
wi·ki - A Web site developed collaboratively by a community of users, allowing any user to add and edit content.
An online forum " or message board', is an online discussion site. It originated as the modern equivalent of a traditional bulletin board, and a technological evolution of the dialup bulletin board system. From a technological standpoint, forums or boards are web applications managing user-generated content..
You can receive comments on any page or post in a WordPress.com blog. Comments are open by default.
If you want a Twitter-like blog then you can use the P2 theme.
You can add users >
Staff have provided help for those who want to structure their blog like a website. > http://en.support.wordpress.com/using-wordpress-to-create-a-website/
What’s important when creating a website structure is the differences between pages and posts > http://en.support.wordpress.com/post-vs-page/
There is only one dynamic page in a blog for posts and we cannot post to more than that one page. We organize our posts by assigning Categories to them. When we publish a post it automatically appears on the running page for posts and also on the Categories pages and Archives pages.
A custom menu allows you to display Categories in tabs along the horizontal navigation where normally only Pages tabs are displayed. If you wish you can also include Pages and custom links in your custom menu as well.
These are the how-to instructions:
1. You create a custom menu. http://en.support.wordpress.com/menus/
2. You add the Categories you want to display and arrange them in the order you want.
3. Then you add and drop and drag the sub-categories below the appropriate Categories, and when you have everything arranged in the menu that you want you save the menu.
4. Go to the “theme location” module at upper left on the menu page. Select your custom menu name from the pull-down labeled “primary location.” Click the save button in that module.
5. Refresh your browser so that it isn’t possibly pulling a cached page, and view see your new menu with Parent pages and dropdowns to sub-pages.