What makes good flash fiction?
Basic skills make for good flash fiction. The mightiest story can be brought low by ignoring the basics.
Good, clear writing is a big part. And not just grammar, but sense. Without it, people are going to have to stop and start, they're going to have to re-read it whether they want to or not. (The goal is to get them to enjoy reading it, and make them want to re-read it.) Ambiguity is a great tool, but far too easily used for the purposes of evil. (evil, in our case, meaning 'bad storytelling')
Remember, you know what's happening, the reader needs your help.
Characters are pretty essential. But that's another statement so broad you could sink an empire in it. You can describe a society or an environment, but the reader is going to associate all the better with someone they can get involved with. There's an argument for the environment or society being a character in their own right, and if you can pull that off then fair play to you, but it isn't easy.
And don't crowd it. Too many characters in a small piece and people will get confused. Characters need to be more than a name, you don't have the space in flash fiction to individualise lots of characters and tell your story.
Get stuck in. Details are key. And this goes side-by-side with character details. Broad strokes are OK for story pitches or book blurbs, but they don't leave a lasting impression. Get involved, get the reader involved. You don't have to describe everything about a person, but one or two distinct features or mannerisms go a long way. State the species of tree, the make and model of car, the building material... 'oak', 'willow' or 'pine' takes no more words, no more space, than 'tree' but does so much more work.
Remember too, that people have more senses than just sight.
The meaning of plot is debatable. I've seen it argued that plot is more apparent in genre flash fiction than literary. Especially plot twists. Personally I see no reason for this to be the case. An interesting character is an interesting character whether they be in the living room or the bridge of a starship. Plot twists are perhaps more easy in a dramatic genre sense, especially horror, but a well-crafted, less-than-obvious plot twist is welcome anywhere. (read some Roald Dahl short stories)
I think the key to plot is movement. In a basic sense, movement of time, but also movement (or change) in character, in the reader's perception. Does your story end up in a different place to where it began?
Genre deserves a mention, but is so personal there is no right or wrong. Flash fiction gives you the chance to experiment, to try genres you might not consider 'your own', or mash-up several genres. The piece is short enough that depending how heavily it sits within that genre it might be readable and enjoyable by someone who wouldn't normally read that genre too, they get to try something different.
If the basics are right, if you have a well-crafted piece of fiction, then the genre doesn't necessarily matter. The setting doesn't matter as long as it feels real. The character can be human, or not, and that shouldn't matter if your writing is clear.
And finally, edit. Most of my stories sit for a few months unread before I go back to them and edit them with fresh eyes. If you have someone to critique for you (and be 100% honest), all the better. Don't be too precious, if that awesome line just doesn't fit, get rid of it.
Of course... I am talking basics. Maybe you want to abuse the system for specific effect, and that can be done really well. Stripping detail out and purposefully simplifying your story can give it a more fairytale feel, for example. Being ambiguous and confused can create a dream-like (or nightmarish) feel. But master the basics first, learn how to tell a story, practice, then start playing, with form and style, with genre, with the reader. =)
If you've read this far, please comment, I'm still learning myself. This is what makes sense to me, this is what I think about as I write. Discussion of process can only improve us all.
I hope this has been helpful to you. If it has, please feel free to share it (but please, credit and link me ;) )