A couple of things I have been noticing is that a lot of content ends up in the TEMP directory of the MOSS service accounts (think content deployment, workflow etc.)
We all know that the default TEMP directory (if unchanged) is in the %userprofile%\AppData\Local\Temp directory which lies on the %system% directory, what I have been doing during setup of MOSS is to ensure that this variable is changed so that any TEMP data is stored on a separate disk.
You will be surprised at how big the TEMP directory can get!
Regards,
Doug “bobTheBuilder” McCusker