Still thinking about this. As far as I can tell, there are three classes of data that I care about:

#1) Memory. I define these as smallish data files which tell applications to behave (preferences), how to configure themselves (bookmarks), and what to remember (think the NetNewsWire preference file which states which feeds are read or not).

#2) Documents. Everything else. Photos, music, documents, logs, mail.

#3) Binaries. Applications and Operating Systems.

The biggest difference between #2/#3 and #1 is that Documents and Binaries are numerous and large. This makes the process of moving them around laborious and unreliable. Memory files are small, they should always fit nicely on a USB memory stick.

Hmmmm....