My motivations in using plain color icons are as follows:
1) monochromatic icons look perfectly in place, because they look in context within this forum structure and make sense when you need some visual help, without confusing the user sight. The section titles are much more important than the icons themselves.
2) The monochrome is less confusing and it's way more helpful than many-colors-icons: this is because the vast majority of users don't look for a forum section looking at the icons, but at the section name.
3) Symbols are there to help us in case of need, and from a physiological viewpoint they must support us when we don't find things in text. The forum structure is built on text, so I personally think that this monochrome approach is the most efficient for our community goals.
4) My viewpoint is that there is nothing more relevant than text on this forum because support, documentation and help for users is the ultimate goal.
5) OS icons are usually perceived to be "as-is", so I would not classify the OS icons on the same level as the other iconset items. OS icons reflect brands; the other icons do not, and in this way they look appropriate. Notwithstanding, these are my personal opinions, and they are the result of years of research, experience and results. As I always suggest to my collaborators and clients, I would prefer making evolutionary changes, rather than revolutionary on the user interface; the evolutionary approach enables users to get confident with new things in the environment while also communicating clearly wether his/her needs are being met. By evolving the interface we add a small new feature (the iconset in this case), and we observe the feedback; we analyse it and keep it or modify it accordingly if needed. Small details make big changes and make UX smooth. When we have more accurate feedback let me have some notice, so we can understand how to move better
PS. On the October 24th I'll give a seminar on Audacity here in Italy, in occasion of the Linux Day. Given that this year's event theme is "Community", I will present the Audacity Community (the software, the forums and the community)!
As always, guys, I really appreciate your effort, patience, interest and time!
Just want to be constructive with my replies, but also objective for what experience and past projects taught me.
Have a nice day,