Echoing @cranebump, as someone who tries to write a story, hoping against hope that it's half decent, I've honed my style, for want of a better word, to several things:
(1) I always use large text (dating from a player with weak eyesight complaining).
(2) I format to leave readable chunks rather than the blank wall of text you sometimes get.
(3) Color coding text so that pertinent information pops at you, plus color coding who is speaking also helps.
(4) If I have to use an outdoor map I will but indoor ones are preferable. Outdoor spawn points are heinous. I tested this with a tiny outdoor map that allowed 5 collectibles. I had 5, and nope, you can't force them to appear in the order you wish, so you literally can't give a hint to a player to help them with a runaround.
(5) Nav bar aid. I like to have objectives delineated where necessary as "Defuse Bomb [Third Floor]". So if you're on the first floor you know you can speed through the second floor.
(6) My initial work doesn't have it but later ones do (might go back and do it) but I've taken to numbering all clues and giving an X to a clue that comes from a non essential glowie. And for clues from each mission to be assigned a different color. So all Mission 1 clues are yellow, all Mission 2 are blue, et cetera,
(7) It's my aim (I don't always succeed) that each boss/clue/encounter met in a mission builds on the story.
(8) I like Easter Eggs! My stuff is riddled with snippets of CO* lore, from the game/comics/novels. So if you spot them and get them, cool. If you spot them and don't, well that's how Easter eggs work TBH.
(9) I have yet to get the hang of writing souvenirs!
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As a player I look for playability, readability, story and consistency.
And if I start running into things like constant fire damage mobs I will just quit. If I wanted to play a fire farm I would.
Other stuff? I think much of what I look for has already been said, save my initial test: If your contact just has the standard AE program data then you're off to a bad start!
Repetition. No issues with that. Essentially all arcs are sleight of hand to hide the fact that you need to defeat a boss/click a thing, but if you write a 5 mission arc and every single one is "Rescue the person", then I'll probably not play another of yours.
And then there are the arcs I play where I think "I wish I'd thought of that!" Sadly those are few, because once you have a reasonable grasp on the tools you have an idea of what can or can't be done.