So, I really respect what you've done (for those who don't know, Chris is the original developer and lead of DifferentialEquations.jl) and use your work heavily. However, understanding and writing idiomatic Julia, especially with these large packages, is severely hampered by the documentation culture.
To be fair, I've only submitted a small documentation patch for a package and haven't significantly "put my money where my mouth is" on this topic. But I hope the next time there are thoughts among the core team about what is the next capability to add to the language, addressing this deficiency is prioritized.
FWIW, I posted the other month that I'm looking for any devs who can help with building a multi-package documentation for SciML, since I don't think the "separate docs for all packages" ends up helpful when the usage is intertwined. SciML is looking for anyone looking to help out there (and there's a tiny bit of funding, though "open source sized" funding). In the meantime, we're having a big push for more comprehensive docstrings, and will be planning a Cambridge area hackathon around this (follow https://www.meetup.com/julia-cajun/ for anyone who is curious in joining in).
As for high level changes, there's a few not too difficult things I think that can be done: https://github.com/JuliaLang/julia/issues/36517 and https://github.com/JuliaLang/julia/issues/45086 are two I feel strongly about. I think limiting the type information and decreasing the stack size with earlier error checking on broadcast would make a lot of error messages a lot more sane.
A prior comment I made, all of which seems unaddressed to me three years later: https://news.ycombinator.com/item?id=20589167
To be fair, I've only submitted a small documentation patch for a package and haven't significantly "put my money where my mouth is" on this topic. But I hope the next time there are thoughts among the core team about what is the next capability to add to the language, addressing this deficiency is prioritized.