So now you have some idea of the format of your submissions and how to prepare them. But what actually makes a good workshop piece? A hard question to answer as there are many X-factors involved: plot, structure, dialogue, form, balance, etc. And it all has to come together cohesively.
However, from my workshop experience last year, there are three criteria which stuck most in my mind: Imagery, Metaphor, and Meaning.
Imagery is vividness. How well can you make your characters, your emotions, and your settings come alive? Can someone reading your story capture in her mind's eye the detail and the atmosphere of what you want to evoke? Don't just write out what's happening, but engage the memory of the senses. Color. Sound. Smell. Texture. Taste. Be sensual, be lush.
Metaphor is subtlety. Don't force the emotion or idea that you want to convey down the throat of the reader. Tell it sideways, with a slant. Let the images you paint carry your message. Like a classy burlesque stripper, you have to hint and tease and tickle. You are under no obligation to reveal all; the pleasure to the reader is in the discovery.
Meaning is, well,....meaning. What is it that you want to say? What essential truth are you trying to convey? What you write should foremost have meaning for yourself, though obscured through the lenses of imagery and metaphor. While you may not explicitly have meaning in mind when you set out to write, meaning should become apparent during your subsequent reading.
The dual pleasure in a workshop is to see others discover this meaning and to discover the meaning others see that you may not have intended.
So there you have it: imagery, metaphor, and meaning. If it sounds too much like poetry, then you're right: these qualities are at their peak for that writing form. However, they also heighten the appeal of short stories and essays. It helps if you apply poetic techniques to all your work.