Coming from a science background, I couldn't agree more.
5/ Arrows are important and svg markers are great. They could be even better if they followed the style of the stroke they are applied to.
Yes, this is something I am personally interested in working on. I've dug into the code and know exactly what needs to be coded; I just haven't had a good solid stretch of freetime to work on it. (The test harness has been a higher priority.) Again, if someone else would like to get involved in doing some C++ coding deep in Inkscape's guts, I'd be more than happy to share my notes and help get you started. If no one else does, I'll get to this eventually, but be patient as it may be a while...
Bryce, I might be interested in implementing this. I've looked a bit at the code but would probably need some help to understand it fully. Can you outline what you think needs to be done? How would the use of style sheets effect this? And SVG 1.2 shadowInherit?
Another change would be to allow scaling the marker size relative to (or independently of?) the stroke width. This should be relatively easy to add if one can get the color to match since in both cases it appears to me that a copy of the marker must be stored in the def section. This would allow the elimination of the three different sizes for each marker type.
As a side note, there appears to be a problem with the current marker code. In looking at what needs to be done I copied the example code from:
http://www.w3.org/TR/SVG/painting.html#Markers
There are two versions, one using markers and one not, simulating what how the marker is drawn. Inkscape renders the two differently. The marker version is wrong according to the picture on the website. FireFox 1.5 renders them both the same. Doing a little research it appears that Inkscape does not use the viewBox attribute.
Tav