How (Not) To Write Flash Fiction

There's been a short story competition going on at Escape Pod, which is part of a series of speculative fiction podcasts. For this competition, the criteria were "science fiction, up to 500 words," and people have been voting on the ones they want to progress over a series of rounds. All of the entries have now been posted and they're just wrapping up the last groups before moving onto the next round of voting.

Some thoughts, in no particular order, about what did and didn't work for me:

• Use the available space. 500 words is a lot different than 100 (citation; have entered those contests too, also did poorly). If your story caps out at two long paragraphs, I'm going to find it hard to match up to someone who made all 497 count.
• On the other hand, 500 words is usually not enough for a lot of jumps/time skips/alternate POV. Mayyybe one *** and then "the next morning, they found the body" or "from the alien's perspective, blah blah." If it's sort of a final-line punch, that doesn't even need the section break. But if you're jumping between points of view rapidly in this kind of space, you're likely not fleshing any of them out.
• There should be an easily-identifiable SF element. That can be near-future (search engines that have too much power) or far-alternate history (time travel for days), but if it boils down to "the perfect boyfriend is like cold fusion, because neither are feasible," that feels like something in our world. Just because you allude to science elements doesn't mean non-standard technology exists within the setting.
• Proofreading is important! If spelling/grammar isn't your thing, find someone who can help.
• This is subjective, but I would say be careful with non-standard points of view. Third and first person are both common; second is a hard sell. Also, and this is not a criticism I would have expected to make before this competition, first person plural is jarring. "We evolved, we rose to the stars, we launched spaceships, we were full of hubris" sounds like the voiceover to the first five minutes of a nature documentary. If you expect me to care about an individual story, you probably want something more "zoomed in" on one character.
• Conversely, I personally find epistolary/documentation/in-universe artifacts to be a very effective way of making a short word limit count. Minutes from a board meeting? Trial transcript? Reddit post? (A few of these were in fact entered.)
• I want to see more to the story than a conceit or idea that is already out in the world. "The Trolley Problem: the fanfic" is not good. Neither is "The Paperclip Monster: the fanfic."
• Similarly, if it boils down to a single political allegory, that's probably not a very good story. "The human was mean and bigoted towards people who didn't look like him, but then, surprise! The extraterrestrials who didn't look like him were bigoted towards him as well!" is not very creative. (That's not to say having political content is bad. Lots of great fiction is political, and lots of great genre fiction is political. But if it's a superficial Aesop, you're probably not telling an interesting narrative.)
• Try not to infodump. Throw us into the world and expect us to catch up.
• Again subjective, but I don't want the language to pull me out of the story. If every sentence is how the vast gargantuan maws of the abyss swirled and devoured the precarious assemblage in its unyielding gravitational embrace, I'm going to be distracted from the characters.
• Humor is good!
• The reader should be able to understand what's going on. If not immediately, then by the end, it should come together (or the last line packs a punch that makes everything else worth rereading). If I'm confused and/or don't care enough to puzzle it out, I might not be inclined to try again when there are ten other stories to read in the round. (With a 500 word limit this is less of a risk, because most people will be willing to reread, but still advice.)
• Personal opinion: I don't like bleakness. (Again, lots of well-regarded (genre and otherwise) fiction is dark.) But I would rather have a hopeful theme than "humans are just doomed" or "humans are just stupid" etc.

Anyway, I didn't advance, so don't take my advice as expertise; I just wanted to ramble about the things I noticed.