A metaphor asserts something to be true that isn't literally so, while a simile merely compares two unlike things. A simile is true, and a metaphor is a lie:
Baseball is like chess. A 100% true statement, and typically, the statement is followed by an explanation of why this is so.
Baseball is chess on grass. No, not really: chess on grass is chess on grass, literally.
But more than being literally untrue, a metaphor literally creates an entire alternate, fictional universe whenever it's invoked. Along the way, interesting aspects of one or both things being compared are illuminated, and some work has to be done by the reader - unlike with a simile, the meaning of the comparison isn't always provided by the author.
The dull glow of the laptop screen stared back at me, daring me to complete the blog post. I turned away, but the low whirr of the internal fans taunted me, calling my name incessantly until finally I turned back to finish the piece. Here, the reader imagines the scene, giving a face to the screen and a voice to the fans, neither of which exist in reality. Strictly speaking, this section is fiction, but it's much more evocative than a straight description. Not only does it describe the scene, it hints at the fact that the writer has been spending too much time with the laptop, and is beginning to think of it as a real person.
The dull glow of the laptop screen was like a face staring back at me, daring me to complete the blog post. I turned away, but the low whirr of the internal fans was like a voice taunting me, calling my name incessantly until finally I turned back to finish the piece. The "like"s interrupt the flow, keep the reader from fully imagining the scene and maintain the strict division between truth and fiction. The blurring of lines that reveals interesting possible truths is totally prevented.
Of course, both simile and metaphor can be used nearly interchangeably (which is why they're taught with so few distinctions). But just because they can be used interchangeably, doesn't mean they ought to be.
I recognize this is all probably pretty obvious, but what are blogs for, if not pointing out the obvious?