Hi all,
Even though I'm not an active user of Inkscape (yet), I do follow this mailinglist actively and enjoy the current discussion on interface design.
There are certainly ways you can judge an interface, as one person pointed out would be hard or impossible to do. Asktog.com has some interesting suggestions for this on the page 'First Principles of Interaction Design' which can be found at http://www.asktog.com/basics/firstPrinciples.html
I especially want to stress the importance of Fitts's Law in all graphical interfaces: http://www.asktog.com/basics/firstPrinciples.html#fitts%27s%20law http://www.asktog.com/columns/022DesignedToGiveFitts.html
My impression of the Inkscape interface is that there's no real common sense about how to create the interface among the developers. I hope I'm wrong about this and that there are guidelines for developing for the Inkscape interface. There are several HIGs (Human Interface Guidelines) available for free on the web, who basically tell the same. I'm a real fan of the Mac OS X Aqua desktop environment and think that the Apple HIG are argumented very well. I'd suggest that at least some parts of the Apple and Gnome HIGs should be used as guidelines for developing the Inkscape interface and should be referred to (at least the useful parts of) at the software development documentation. This only is useful when all developers see the relevance of this and consistently apply it.
To start with streamlining the interface, I want to suggest to take a look at what Apple and Gnome have to tell about icon design. Though they disagree on the style of icons, they do share a common thought on functionality and consistency.
Apple: http://developer.apple.com/documentation/UserExperience/Conceptual/ OSXHIGuidelines/XHIGIcons/chapter_5_section_1.html#//apple_ref/doc/uid/ 20000967 (that URL is far more user-unfriendly than the text itself fortunately)
Gnome: http://developer.gnome.org/projects/gup/hig/2.0/icons.html
I hope my suggestions were of any use, though I hope even more that it was old news...
Regards, Lars Boelman