@Zagna - The translations won't really be affected, you'll still be able to copy the text and put it into the translation tool of your choice. There is also the built-in link to the Live translator, which will be maintained (probably just moved around a bit to make it easier to get to). And yes, it will be valid HTML and CSS.
@BoltBait - Absolutely, it's all about making it easier. I'm currently auditing all of the help content and trying to clarify parts that are unclear, but any additional input is greatly appreciated. Are there any places in particular that you find vague or confusing?
@Goonfella - Since the plugins are developed by the community and are not an "official" part of Paint.NET, we can't provide support for them. In addition, providing links to certain plugin packs gives them greater exposure, and we want to keep everyone's plugins equally accessible, so the plugins will need to stay on the forums which are open. That being said, I think that a link to the plugin forum in the help documentation is a great idea, I'll add it to the list. I'll also talk with some of the Mods and see if we can't get some of the plugin developers to write tutorials on how to install their plugin.
Great suggestions guys, please keep them coming!