Search for things like “rich editor” and “wysiwyg” and you’ll find a lot of the old discussions covering this subject. In short, it is a seemingly simple problem that is incredibly difficult to execute in practice. It is for good reason that most popular social networks pretty much stick to plain text.
That said, “enhanced plain text” is something I’d love to experiment with, pretty much on the same level of Facebook, i.e. rendering oneboxes and images in-editor.