Being able to run PHP scripts and applications is probably one of the most obvious feature in shared web hosting today, but the actual process to make it fast and secure at the same time might not be that trivial.
I am actually writing this post in response to this thread at WHT, where web hosts discussed ways to fight back against those 200Gb oversold plans that are populating the shared hosting industry today. That thread went off-topic half way through when Matt Heaton of BlueHost and Kevin Martin of Pair Networks had a not-so-friendly discussion on whether to run PHP as CGI or as mod_php on servers.