Its not the specialness of the mac that makes it "more resistant" to viruses, fact is its simply security through obscurity. There is no reason to develop viruses for 10% of the computers when you can hit 90%.
Same thing applies to firefox (called pheonix at the time), when IE owned 90% of the market there was rarely any security issues..now that it has become a force there are security vulnerabilities found left and right.
Many take security through obscurity to relate to OSs with low market share. But, security through obscurity applies more to knowledge about an OS's design and implementation. The parts of OS X that make up it's kernel space elements that relate to virus install are open source. Any one can download the source code to see how it is designed and implemented. By definition, open source functions in the exact opposite to security through obscurity. Proprietary code relies on security through obscurity by not making the source code available. The kernel space parts of Windows are mostly if not all propietary code.