Hi Georg,
Unfortunately, that is a tough question to answer. And there are most likely many answers. I can only speak for my self by saying, being a Test Engineer myself, I am a fanatic when it comes to documentation. In my line the better the doc the less the customer issues I have to deal with. But I am aware you cannot make someone read. And you can get yourself in over your head trying to keep up with it.
So I say documentation is essential, and every feature does need it. The LIVE feature that many tools (but not enough) have is really nice. I can look at the doc, as I am working the image. This is very handy.
The first tool I learned to use, (well almost the first)was the DBE tool. This helps out with my LP issues. But in reading the doc, (which is fine by the way), was confusing, because being a complete lamen at this, when I read the mouse over explanations, I had no idea what it was saying. I knew nothing, and still am learning about sigma units, mean background, 2D-surface spline, etc,etc,etc.
So in many cases more elaborate documentation isn't that helpful. But for some I am sure it was great info.
One thing that would be nice is some kind of doc "explaining a fuller view of the concepts of astrophotography and PI" as you said. . But I think it should include some simple items like DSLR formats explanations like the ones in the "Format Explorer", but more verbose. At first they meant nothing to me. Now I have somewhat of an understanding, the the 3 bottom explanations in the top section make sense. But I have no idea what the top one (Create super-pixels mean). It's things like this, that I can't expect you to document, I just appreciate the answers that I get.
Another thing that goes with the format is the "hints" in some tools. Had no idea what they were for. Even after reading the description. Do now of course.
I just don't have an answer for you, except docs for each tool. Obviously a book like those from Jerry Lodriguss on PI would be great. But that is a heck of a chore.
One feature of PI, that I am just learning to use, are all the scripts. Lot of talk about the mschusters scripts in the Scriptbox.
But once again, if you don't know what the science is behind it and understand what the parameters are to be and what to expect, the tool doesn't makes sense. Also even how to download a script and execute it properly is not explained. I have run into dependency issues when trying to execute some scripts.
My biggest gripe (and this a gripe to myself), is I do not know what to expect for numbers on my images. I am trying to learn about FHWM, noise values, sub exposure estimates, I could go on and on.
I would appreciate someplace where I could get some amatuer and non-professional data images to open with some tools and get ideas of different values. But I don't know if just a sub or just the stacked image would do, or possibly the final processed image. But I would like is that I could open and compare to learn more about what expectations or goals I could look forward too. I know my data is not the best, but I have no idea what good data numbers look like for some of the tools and scripts described above and what I should shoot for.
So you see, I think just knowing that there is someone , and usually several users that can answer questions as they arrive, is the most important attribute and most invaluable feature of this forum, which as far as I am concerned is part of PI.
I think you said it all when you said "The technology behind PI is amazing", however that means it is an advanced piece of software, and will require a lot of educating and learning.
Scott