Dear Axel,
We had started the LAMMPS category in the MatSci discourse forum with the expectation that a forum might encourage participation of a larger number of people better than the lammps-users mailing list. Sadly, that has not happened.
- Operating and moderating the mailing list and the forum at the same time is not going to work forever as it requires duplication of efforts. What speaks for closing the mailing list, what for making the LAMMPS categories in the forum read-only and only use it as a mailing list archive?
That might just be me, but I wasn't even aware that the forum was supposed to replace the ML. Just very subjectively, active reading seems to be more work than getting mails delivered and skimming for important ones, but I'm well aware that this is a dying mindset and people love their web platforms 
What are the reasons that people do not participate in discussions, even if they would know (part of) the answer?
Interesting point: I used to be a moderator on a developer forum until a few years ago, and we had very similar issues. Fairly open communities that attract professional experts as well as students seeking homework assistance seem to always have a weird signal-to-noise ratio. It is surprisingly difficult to motivate oneself to engage with the "low-level" questions when it already feels like a basic level of due diligence was not done.
You had already discussed that in your reply to Emerson, but I wonder now if this is more of a visibility thing. Some questions can be answered by Google use or are so fundamental that they're not even LAMMPS don't really belong here, but that would probably be easier on MatSci, just tag them as something else.
- As a developer working on software to be added to LAMMPS or writing a software that uses LAMMPS as a library or that is working on modifying LAMMPS, where do you see the biggest deficiencies? What would need to be done to make your development work easier?
- As a long-time LAMMPS user or developer, which of the many changes that we have made over the last 5+ years have worked well for you? Where did LAMMPS go backwards and should revert to how things were done previously?
Well. "Long-time" is relative, but to my surprise just now it's been almost 7 years since I was taught LAMMPS as part of a lecture. Time flies, huh.
Let's say this: my PhD thesis heavily relies on LAMMPS and my contributions to it, MEAM etc. In the draft's Acknowledgements section, I write this: "The collaborator/community management by [you] is among the best I have seen on any open-source project, both personally and on a technical level."
So yeah, I'm definitely happy with the structured way of contributing and all of the code quality improvements that come with it. Some of the old code is... not easy to read, even with the updated programmer's guide. I'd suspect the entry barrier for further changes does get lowered, which is nice.
Doesn't answer a lot of your questions, but maybe helps a bit for some 
Best,
Sebastian