Amateurs self-select for the job. Professionals are selected. For most jobs, volunteers beat draftees.
No matter how much you love your job, you will eventually end up doing something that feels like work--something that you have to do because your boss asked you to or because the market requires it. At that point, your professional skills may be negated by your lack of authentic interest.
But amateurs are by definition volunteers. They choose to spend their time on what they do, and they go exactly where their passions, interests, knowledge and personality takes them--no further. If they lose interest they move on and are replaced by someone bursting with fresh energy. Self-selection ensures engagement.
To me that's the difference between amateur and professional content: the first may not be polished, but it's driven by the sort of intense interest that cannot be faked. The second may be better written, spelled more correctly and otherwise competently produced, but all too often it has the arms-length perspective of a drive-by.
This is one of the problems with professional journalism: journalists go where the story is, and every day brings a new story. Journalistic skills are portable, but deep domain skills are not. Meanwhile, the amateur lives one story, their own. They make lack journalistic skills, but if you're interested in their world, there's no better guide than a native.
These days I find that more and more of what I read is created by amateurs (or at least amateur writers) talking about what they're most interested in, whether it's Spore walkthroughs or robotics tutorials.
I'll take a passionate amateur over a bored professional any day.