Marco Tabini has posted his list of what he considers five features of PHP5 that you "can't afford to ignore" when doing your development work:
Despite the fact that you may not have a choice in the matter, upgrading comes with a number of bonus new features that can help you write better code and gain access to new functionality that required a fair amount of hacking in previous version. Here's a quick list of 5 personal favourites.
The feature to make his list are SimpleXML, JSON/SOAP, PDO, the Standard PHP Library and SQLite. Each has their own bonus feature(s) included too for a little extra incentive to check them out.