I think you need to realize that the primary
purpose of the Kindle is for it to display books obtained from the Amazon store and that is what they concentrate on. It is not, and Amazon has never claimed it is, a digital Reader that will read every document ever created. It does display some other formats - not all of them - but some. God knows it doesn't display ePub as we've all read 3 bazillion times.
If what you want to read is books that you already own in a different format without converting them to Kindle formats, then the Kindle probably isn't the optimum digital reader for you.
Kindle - (Mobi, PRC, AZW) .txt, PDF. There is a document conversion service for DOC, html etc but I find calibre does it better.