Losing mail is an extremely bad thing, if you are managing an ISP or major hub. Think about RAIDing your mail spool and consider frequent backups. Software development can require a large number of directories for binaries, libraries, include files as well as source and project files. If possible split as much as possible across separate drives. Web browsing is becoming more and more popular.
Many browsers have a local cache which can expand to rather large volumes.
As this is used when reloading pages or returning to the previous page, speed is quite important here. If however you are connected via a well configured proxy server you do not need more than typically a few megabytes per user for a session. See also the sections on Home Directories and WWW . We could make all 4 directories separate partitions but would lose some flexibility in managing the size of each directory. A better solution is to make these 4 directories symbolic links to appropriate directories on the respective drives.