I thought i'd share my thoughts regarding your points above, bearing in mind i'm not a theme or plugin developer.
I've not used an abundance of plugins, but i've still tested a good handful of them, some have worked, others not so. Success rate i'd say for me is around 60/40, the 60 being success. Assuming i've checked the plugin version, checked compatibility and followed install instructions it's usually an easy process. Of course that's not to say they all work out well, some just cause errors, won't install or are totally useless once installed.
There are various ways to add options into the admin section, and i'd personally attribute the placement of options on 2 factors, firstly author preference and/or secondly the level of coding skill the author has. There's no GOLDEN RULE that author's follow, so everyone will naturally have a different interpretation of how things should be coded and placed.
Totally agree with you on this point, however i can't honestly say it's fair to expect each author to publish X amount of instructions, and where does the line get drawn, what's sufficient for one person may not be for another... ALL THE SAME... agreed, there are some serious issues with docs, either too vague, too little or impossible to interpret, or non-existent.
This is such a subjective issue it's hard to say where the line should be drawn in terms of required information. What makes total sense to me, may be like reading chinese to you.
Too big a topic and discussion to go into.. if it were me, i'd code with FF and Opera in mind, and work out IE problems last.
I think on the whole every author has a different version on how to code, what to document and how much effort to put into explaining their code uses.
Plugins and Themes listed here are created by people just like you and me. The community is open and free, you can't expect perfection from something free. Each author has his view on how to do things, if you don't like how they handle an element of their work, then don't use what they create.
Any script, design or function available for PHP & HTML can be intergrated into WordPress. Just because it's not written into a plugin or easily pluggable line of code doesn't mean you can't set about doing it yourself..
Can i find everything i want on my WP install here and ready in a plugin or theme for me? NO!...
What do i do?... google for a script that does the job, hack it up, and intergrate it into my WP install.
For example i'm currently re-writing various pieces of a plugin to convert it into a new feature for my site, testing locally. This is the alternative to starting my own one from scratch. I've no intention of releasing it as a plugin, and it's coded with that in mind, as in "me" ... i'd think if safe to assume other authors may do the same from time to time, code and design first, documentation and ease of use last.
I think if i were intending to release it as a plugin i'd proberly just write something brief then direct additional support to my site, as like i said, my time is best spent on the code as much as i like to help people i'd still opt for directing my time on writing code..
I don't think it's really fair to expect a particular level of any kind from developers of FREE code. It's free, they're not under any obligation to explain how X functions as Y because the person attempting to use the code doesn't have time to learn how it works by themself.
I'm not suggesting that everyone go learn how to be a PHP guru, but i think it might be a little hypocritical to ask authors to put additional time into something they've already spent numerous hours/days/months on, when you/me/joe bloggs is trying to do just that, conserve time..
From a coder point of view (whatever level that may be) i'd say the most fun is in the code, not spending time writing docs on how to use or understand the code i've written (though sometimes fun)... So i can understand why some devs may not feel any need to put much effort into the docs.
Not sure how that came across, i'm not really one for long thoughtout posts... :)