When you open a .doc file from an older version of Word in Word 2013, the label "[Compatibility Mode]" will appear after the file name in the document's title bar. You can convert an older document to the new Word 2013 mode and to the new .docx format, but doing so is undesirable if the documents belongs to someone else—especially if that person is working with an older version of Word.
To know which specific features Compatibility Mode is blocking in a given case, you must know which version of Word the document is compatible with. To find this out, choose File > Info > Inspect Document > Check for Issues > Check Compatibility. Next, click Select Versions; the version of Word that your document was created in will be checked. Microsoft provides extensive details about the process online, but couldn't the company display this information in the document title bar?