Choice is one of the defining hallmarks of Linux, and that includes not just the choice of which distribution you use but also what desktop environment you prefer. So, you're free from the dictates of any single OS maker and can customize your experience pretty much to your heart's content. Fans of Ubuntu who don't like its new, default Unity interface, for example, can simply swap in something they like better.
3. Superior security
Thanks in large part to its ubiquity, Windows is notorious for the viruses and other malware it tends to pick up; Macs, apparently, aren't a whole lot better. Linux, on the other hand, is renowned for its superior security--so much so that security experts recommend it for sensitive applications like online banking, for instance. There are even extra-secure distributions available for those focused on security.
4. Modest requirements
Whereas the Windows ecosystem is often referred to as a "treadmill" for the way it forces users to keep upgrading their hardware so as to keep up with the software's requirements, Linux is quite the opposite. In fact, it's long been embraced for its ability to run flawlessly even on older or low-resource computers. Wouldn't it be nice to keep your existing hardware a little longer and put that money towards something else instead?
5. Open and free
Last but certainly not least is that Linux is totally free and unencumbered by license restrictions. You can download, use, and customize it at will without fear of the wrath of any vendor. You're free, in other words, to make it your own.