Organized, accessible assets are important part of every project. Still, rather to impose yet another asset system, I would like to share idea of improved File manager based on thumb previews and similar features already build-in. In this structure of folders, files could present a single asset or projects itself. In order to make this system working, we should also integrate similar functions of appending, linking and opening into single - file browser window. By that we could also choose what would happen if we drag the object from the panel to the scene. These features alone should be interesting enough whatever you like the idea of assets or not.
The idea is also filled as a proposal on Blender Wiki under the Quick hacks section.
You can also visit my Wiki page with a better description alongside some others suggestions:
That was so far my contribution, based on my current workflow. In any case, feel free to comment.