WordPress is a powerful publishing platform (this site uses it) and it seems to be going from strength to strength. This is partly due to the fact that WordPress is easy to hack i.e. it’s easy to re-write code to make the platform do what you want it to.
Hacking core files, however, is problematic as the hacks are overwritten whenever you upgrade to a newer version, something that you need to do fairly often due to security holes being found and plugged and new features being introduced.
To overcome this problem, read this Smashing Magazine article on ‘hooks’ and how they can be used to provide you with powerful tools to customize your WordPress site.