Mathieu,
to second what Pau said I think that while we appreciate the effort here, multi-colored icons that are not necessarily from vector (vs raster) based sources really increases the overhead to maintain the visual system.
Single color icons that are resolution independent that we can scale, modify, and display easily on desktop and mobile are much more useful.
Depending on design direction, single color "metaphor" icons and easily change with the look of the site over time, and don't necessarily have a strong look & feel of their own, this gives the foundation and designer that help us out more leeway to explore different design options.
We also would love to see greater consistency, if you look at the icon guidelines for Windows, OS X, Android, or iOS you will see cohesive systems for color, perspective, shape, as well as suggestions on how to deal with certain metaphors and concepts. Because many of the assets will be created by the community having guidelines like this is very helpful, and i'd love for my team to eventually create these. The icons you've shown as part of Nuvola are very creative and represent a pretty high barrier to entry for someone with basic image editing skills. If like Pau suggests we utilize the simple flat or subtly volumetric style found on the Noun Project the barrier to entry feel much lower to new users wanting to contribute.
Not to say that creating simplicity is simple, but I think it is a good place to start.
Thanks again for your contributions, check out the Noun Project and let us know your thoughts
Jared