There are three degrees of comparison in English.

The most overt comparison is the analogy.

The windows of a house are like the eyes in a face.

In this presentation, we're rather ponderously pointing out that 'windows' are to 'house' as 'eyes' are to 'face'. Note that all four elements in the comparison are present: windows, house, eyes, face.

The middle level of comparison is the simile, which always turns on the fulcrum of 'like' or 'as'.

The windows of a house are like eyes.

Or, alternatively,

Windows are like eyes.

The most subtle form of comparison is the metaphor.

The windows watched Harding approach.

Here there is no explicit comparison between windows and eyes. The comparison instead is suggested by the verb 'watched'.

The focus in this article is on the middle level of comparison, the simile.

The problem with similes is they conjure something outside the context of the story itself.

His head was caved in like a busted pumpkin.

The reality presented in the sentence is someone whose head is caved in from a beating, but then we're transported to an image ('busted pumpkin') which doesn't occur in the reality of the story. Since this sentence is meant to present something horrific, it has to be noted the sentence moves from a truly horrific image, a head caved-in, to one less so, a busted pumpkin.

Let's look at another bad simile.

Teeth littered the concrete like white pebbles on a beach.

While it can be argued teeth on concrete is visually similar to pebbles on a beach, the simile fails because it removes us from the violence of the littered teeth, the obvious point to the sentence, to the associative calmness of a beach.

Similes, because the very nature of their comparison removes the reader from the context of your story's action, should be used carefully. Even a good simile can undermine the reality of your story, by removing your reader from that reality, even if only for the length of a few words. Be especially stern with similes that evoke a comparison of images (the cheapest type of simile), as opposed to similes of sound, taste, texture or smell.

Similes work best if the comparison created by the simile is more intense than the in-context word or phrase to which the simile relates.

The pumpkin was caved in like a busted face.

Especially with similes based on images, a further elaboration of an image, which remains within the context of the story, is almost always more powerful than a referral outside the story to an analogous image.