Oh, totally agreed. If I recommend one story a day--and I give a recommendation to anything that I actually enjoyed reading all the way through--that's...usually less than 10% of the stories I've read that day. And "like" is a long way from "loved". If I'm recommending anything that weights at 6 or higher on a scale of 1-10, well, there's exactly one story that hit a 9 for me in my reading so far.
Yeah, I do about the same. Out of 10 stories, there's generally only around 1 I actually enjoyed/thought was solid enough to be passed on. But it's still generally something enjoyable, but ultimately forgettable. That doesn't mean that it's a *bad* story. But it's not awe inspiring or extraordinary, either. If you can write something extraordinary, you're almost certainly going to publish it, as long as you submit it a few times, to appropriate places. But there's also space for "good, but not excellent stories". Just only so many spaces, with more stories than there are spaces.
But it's also a matter of variant taste. There are some tropes that will always make me roll my eyes. ("Oh, look. Yet another story in which the protagonist is a horrible person who comes to an ironic bad end. Haven't seen that since five stories ago!") And other people who haven't burnt out on that trope might find other aspects in the story totally make it worthwhile. So a story that reads for me as "Great prose, clever worldbuilding, snappy plot twists, but I hate the whole premise" might be outright GREAT to someone who doesn't hate the premise itself. It's why I like that there are multiple perspectives on these things.
Yup, there's always some taste involved. But a fair amount of skill, too. I never reject something because it wasn't "to my taste". (Although I suspect the head editor does.) But I will reject because there wasn't enough tension, the trope is overdone, major logical errors, whatever.
Also, I think in general slush pile readers are a jaded lot. We've read a lot more short stories than the average person ever will, so tend to care a lot more about things like "over used tropes" and get a lot more excited about really new ideas/weird concepts.
One of my friends started whining about how one of her stories was picked up by a certain magazine while another wasn't, despite that she thought the first was better...and I'm fairly sure the second made it only because it was more unique. (And neither was bad. Neither was awe inspiring, either. But purchased story was both pretty good and unusual, while unpurchased story was marginally better, but more typical. I suspect that someone who didn't read a lot would prefer #1, but a slush reader/small magazine editor would prefer #2.)
And I agree on seeing how hard it is to write a good story. I've learned a lot about why some of my stories fail by seeing a lot of other stories fail in similar ways. ("Oh, so that's why that ending wasn't satisfying!") Reading slush is really useful for getting more data points, so that pattern recognition can kick in sooner.
Yeah, I feel the same way. Reading slush has given me a lot of insight as to why certain stories are getting personal rejections or acceptances, while others seem outright ignored. As you said, it's data points. And lots of them. It's a pity that more writers don't get the chance to prowl around slush piles. (Although agreeing to beta read a lot, or running through a popular critique site gives you similar insight.)