I recently did a lot of Exports to Batch Files in order to handle special stuff with external programs, and for automated cover image handling (using ImageMagick).
I started out writing a Process.bat batch file that actually handles the data the export generates (and takes care of OEM character set conversion). This file is always the same and I stored it in MP3Tag’s program directory.
For creating the (temporary) Process-Data.txt batch file (which gets generated by MP3Tag’s Export), I’d have it also stored in the same place, but of course this poses problems for Vista users (and if more than one user uses the same computer). So I’d like to store my temporary files in the system’s defined Application Data path, defined by the environment variable %APPDATA%, i.e. store it as
%_appdata%\Mp3tag\data\temp\Process-Data.txt
which the system would reference as something like
C:\Documents and Settings\moonbase\Application Data\Mp3tag\data\temp\Process-Data.txt
and which I could reference in my Process.bat »main« batch file as
%APPDATA%\Mp3tag\data\temp\Process-Data.txt.
All this would allow for a safe, XP- and Vista-compatible, easy way to handle Export-generated batch file stuff from within Mp3tag. One could even (as I did), put a little icon »Process Mp3tag-generated Batch Export« next to the Mp3tag icon in the Quicklaunch Bar — making for a two-click »DO IT!« 
For all this, I’d like to ask for a new Mp3tag variable called %_appdata% that resembles the environment’s %APPDATA%.
Thinking about it a second time, maybe it would be wiser to use the system’s TEMP path instead? (%APPDATA% is usually a »system« folder, not all users know how to »unhide« it.)
Maybe we could get both %_appdata% and %_temp%?
What do you feel about this?
... purists would kill you 
- that's the daily business with environment, users & rights. 
) Otherwise we wouldn’t need batch files …