I would also love to have folders in OpenGoo. I don't like tags for folder management because I consider folders to be like categories and tags like well... tags.
The difference being that with folders I can specify topics. I do some web design so for me A workspace would be "Design for Clients", inside I would have folders for the "Micro Design" where only a few hours of work are needed and "Large projects" and so on.
I would use tags to go deeper and tag them with "graphic design", "php", and so on. i would love to have folders, this may be a "management method error" on my part, but I just like them more.