If you are evaluating DotNetNuke, take a look at version 4.9.2. Install it and start playing around with it yourself and form your own opinion. It is very important to consider other's opinions, but in the end you need to decide for yourself, and preferably you reach a decision after actually evaluating the product on your own.
If you're curious what others have actually implemented with DotNetNuke, check out dnnGallery.
I plan to create a community website
aimed at publishing ASP.NET articles,
blogs, forums and video tutorials.
There are several article management solutions for DotNetNuke that you can investigate further. Two I have had experience using are Ventrian's New's Articles and Engage: Publish. There is a core blog module and a core forum module as well (core meaning that it ships with DotNetNuke). There are also third party solutions such as Active Forums and often people use the article management modules mentioned above as blogs.
An editor that lets me post code with
ease
DotNetNuke uses the provider model for most everything, and the rich text editor is no exception. I believe Telerik has a rich text editor that you can plug in, but DotNetNuke 4.9.2 ships with the FCK Editor 2.6.3.
Open source preferably written in C#
DotNetNuke is open source and has a very liberal license that allows you to do most anything with the code. The "core" is written in VB.NET but frequently extensions (even some of the core extensions) are developed using C#.
Easy to export content if there is a
need to switch CMS
While this isn't the only way to export data from a DotNetNuke site, DotNetNuke has a templating system that allows you to export at the portal, page, and module level. If you export data at the portal level you get a lot of information (site information, site settings, page structure, page settings, module information, module content, etc...). This could be used to import that information into another system, but would require you as a developer to parse the XML that was exported and store that in your new application.
Also, in the US, what hosting company
would you recommend to host this site?
Hopefully, it won't cost me a fortune
to maintain it. Should I consider
DotNetNuke as some hosting providers
have DNN installed by default?
Here are some links for more information on DotNetNuke hosting.