Google announced that as of the end of this month you will only be to export as .docx, .xlsx, and .pptx files. Those are the formats that are supported in the current version of Microsoft Office. If you are still using Office 97-2003 Google recommends installing the Office Compatibility Pack for Word, PowerPoint, and Excel.
To clarify, this change applies to the Office formats Google Drive will allow you to export to. You will still be able to export content to PDF, Rich Text, Plain Text, .html, and Open Document formats.