Perhaps this part explains all the commotion around rust... being a popular new language with strong opinions, but today there is a lot of entitlement going on in software communities, assuming that it makes sense to integrate inclusivity to such a degree that it's actually incompatible with what makes a cohesive opinionated project.
I'd generalise it: There is a lot of entitlement going on right now.
I've contributed to hundreds of projects, and maintained a ton, so i feel this pain, so please take what i say in that vein :)
The question i usually ask when i see this is: "What is the thing you did that you think should have caused people's behavior to change?" Assuming you want this to be different, what actions are you taking that should have changed this? If they didn't, what did you learn and try?
I'm not suggesting you will get to some perfect place where all contributions are great, but if you really want to improve them overall, you have to be doing things to improve the behaviors you see, seeing whether they work, and iterating, until you get the behavior you want out of most contributors.
"90% of everything is crap." -- Sturgeon's law [0]
The problem is that once someone has put in the time to create a PR, declining it is never going to feel good for the contributor.
Yes, they can maintain their own fork with their changes. But to be told "this doesn't work for our project," no matter how politely, is often emotionally interpreted as a form of rejection.
Which is what spawns the kind of rant above in the original link.
I would also agree you are going to get some amount of crap no matter what. But, i would say if you want this to be different, it's not entirely on other people. Even with the best of intent, you can't expect contributors or teams or processes to behave magically just because they should. You usually need to motivate/incentivize or train the behaviors you want, and iterate the processes and your approach until you are getting what you want.
After all, it seems like their incentives should align? Like, why wouldn't they want to make a contribution that actually helps? Making that easier to actually do seems like time well spent for everyone.
Ironically, my experience overall as a manager (not just in open source) is that when you ask this question, a lot of the time you get the answer that nobody did anything, because of the latter thing you describe.
IE nobody is acting malicious, but they expected people would want to do X on their own, so nobody tried anything specifically.
In those cases, once you get people onto the path of specifically trying things, and seeing what happens, it's often a great improvement. Even without lots of other process/measurement.
Beyond that, one of the things we do internally, that is a bit harder to do well externally due to social norms/etc, is ask people. In open source, it's trickier because if i contribute randomly to 200 projects, i don't want every single one of them spamming me directly to find out more about my experiences ;)
But occasionally poking people on PR's that are taking a while, or went really well, etc, and seeing if the people they are willing to fill out a form anonymously or something, even if heavily biased, would at least give you some notions of what is going well or not for people.
There is also a lot to be said for analytics on PRs, but again, open source is not very advanced at this right now, so i don't think it's near as viable yet as just asking some people
That takes work, sometimes quite a lot of it, and there's no trick around getting the work done any easier.
A lot of contributions to open source are simply worth what you paid for them, which is nothing.
Early in a project it is possible to find areas of the codebase where easy things haven't yet been done because there just haven't been enough programmer-hours done yet to burn through the simple stuff. Late in a mature project the easy things have mostly gotten done and finding more easy stuff becomes difficult, and as you pull apart simple looking issues they start to look hard.