If you put your Mac onto the internet after enabling all those ports (that ship closed), enabling every kind of sharing, remote login, turning off the firewall, have a weak (crackable) username and password - someone will probably be able to get into your system.
Keep those services off when you're not using them, keep the firewall up and you're, so far, OK. There is nothing 'in the wild' that can install itself with the user unaware and let a hacker own your machine.
Equally, if someone has physical access to your Mac, they could do damage - particularly with an OS X boot disc so they can change passwords etc. But again, that's true with most systems.