Be wary of any sort of bias or slant - political, religious, etc.
Are all sides of an issue represented?
Is the author affiliated with some organization that would have a vested interest in swaying your opinion?
Is the language calm and neutral or inflammatory?
Check for the author's organizational memberships - Southern Baptist? Nature Conservancy? the NRA?
Was the source created to inform you about something, or persuade you of something (including persuading you to buy something)?
There are times when it's appropriate to use sources with a bias. When you're examining a controversial issue, you'll want to be aware of what folks on both sides are saying. But regardless of what you're trying to do, you always need to be AWARE of any bias in a source.