* OS X has reached a level of usability and polish to become more accessible by the mainstream, whilst retaining features that appeal to power users. Around ~10% worldwide share and steadily rising.
* High quality hardware (especially units like the Air and the RMBP), with attention to detail and usability. A flood of inconsistent and sometimes inferior PC models, at sometimes equal or higher price points, leads to buyer confusion and remorse.
* A reduction in the disparity in prices due to economies of scale / manufacturing advantages. Whether by spec sheet or by user experience, they are still competitive.
* Cross-platform development by many commonly used software packages. Easy to use VM or dual-boot solutions for others.
* Flow-on effect of the vast numbers of iOS users towards OS X, where Apple services such as iTunes and iCloud are tightly integrated with the OS.
* Gradually increased enterprise adoption of OS X, with much work and documentation completed to assist in AD, network integration and security policies.
* People realising they spend much of their time within a browser, regardless of OS.
I also firmly expect that the introduction of Windows 8 will send many more users over to OS X. Apple can thank Microsoft for sending me over to a Macbook. Review and thoughts upcoming.