I think that the problem with tags is the lack of intrinsic hierarchy. While you can flatten the hierarchy by enumerating all the tags, this approach is boring if done manually or requires an accurate ontology if done automatically. Another risk is a result from the paradox of choice. Which tags are appropriate? Did I use the same word as before? Will I be able to remember it? With folders you are able to further refine your lookup, and a document must be in a folder anyway, while it can lack a tag. Of course these are just points that can be overcome in a way or another by a good implementation, but folders, symbolic links and tags together are IMHO a better alternative that tags alone.