Why it's annoying:
Putting all the content of an e-mail in the subject line and having nothing in the body is annoying because the recipient probably has to keep scrolling to the right just to see what you're trying to say. Plus, most people glance at the subject and then expect to get the meat of the e-mail in the body. If there's nothing there it's just confusing.
把所有该放在正文里的内容全塞进主题行确实是相当可恶的做法。接受者可能要把拖动行一直拖到最右边才能读完主题栏里的内容。另外，大多数人只会瞟一眼主题，然后直接看正文，如果发现正文一片空白，他们会不明所以。