Nice summary, Tim!
Many people have been asking about the state of Stack and can't wait to see it ready for moodle 2.X.
To 2), 4) & 6) we do have many such tools available - Geogebra filter, AsciiSVG plugin, Google Chart Wizard plugin, GeoNeXT/JSXGraph plugin & filter ... or Wiris graphs - see http://moodle.org/mod/forum/discuss.php?d=182627
They are just not available in core moodle and need to be installed separately.
During the past year many sites have started to use MathJax instead of / in addition to Tex filter because... well, you can read some of these reasons in http://www.mathjax.org/
Personally I use both Tex filter with a full ditribution of TexLive and ImageMagick and MathJax on my test sites to render maths with different delimiters. Both have advantages and disadvantages.
There is however one major issue in the current integration of Dragmath (equation editor) - some years ago Petr wanted to add to init code of core tinymce a piece of code that checks if tex filter is enabled and adds the button for dragmath only when tex filter is enabled. Marc strongly protested this since tex filter is only one renderer among many others, you could as well use Dragmath with Mathtran filter, Asciimath or MathJax with script tags in themes or in additional HTML in site administration. And my compromise solution was to create an additional TinyMATH editor, a normal clone of tinymce with different init code and different demo plugins.
Adding plugins or filters to moodle 2 is easy and it is not a big problem to edit lib/editor/tinymce/lib.php to add more tools for maths or other purposes but hacking core files is always a worse solution (no matter if you add Wiris tools or JSXGraphs or what-ever-plugins) than an administration of settings of default editor in administration menu that could allow adding new /changing old init code directly... or workarounds like http://moodle.org/mod/forum/discuss.php?d=193064