I agree. For certain applications, strongly hierarchical organization is appropriate. I think it's probably best if they're separate per-application hierarchies

I agree. For certain applications, strongly hierarchical organization is appropriate. I think it's probably best if they're separate per-application hierarchies. The One Tree file system model is really aesthetically appealing to us hierarchically-structured thinkers, but most people don't seem to think that way. Interesting to note how it's done on the iPad: Photos, for example, are stored in a hierarchy (in certain views at least; there's not a strong sense that the "true" scheme they're stored in is an hierarchical one), but no folder metaphor is used. It's just stacks of images. Likewise, the "folders" in OS 4.0 are that in name and function only. They are represented more like the stacks of photos, though the objects "in" them are shown as tiled (with a linear view when tapped, for easier picking) rather than overlapping, though, in that case. But an opaque container isn't used. So, in both cases, the objects are collected without the container being drawn attention to. This may limit the number of levels of hierarchy that are possible (stacks of photos within stacks of photos would break the metaphor... not that people put real-life folders within folders very often), but most users don't like deep hierarchies anyway, and the extra hierarchy of objects being stored "within" apps helps mitigate that as well as segregate things somewhat naturally by type. Of course certain tasks will always require deep, complex hierarchies.