Bullets of a list are exported with a character which is not proper for plain text.
The indentation of the bullet is also not exported.

As a workaround, the incorrect bullet character could be replaced with something more suitable. Here is an example:
TxtFormatProvider textFormatProvider = new TxtFormatProvider();
exportedText = textFormatProvider.Export(this.radRichTextBox.Document);
string newText = exportedText.Replace((char)61623, (char)8226);