There are lots of new features added in Mac OS X Yosemite. Some of it requires newer versions of your Mac hardware for it to work (So, please check it out first). They say there are reasons to upgrade if only you have iOS devices. But, I urge you to upgrade since it has added security features, bug fixes, faster performance, and revamped User Interface, which I say much simpler (That came from iOS). You must note that some applications may not run and you need updated versions for it to work. Fortunately, the apps you bought in the store will offer updates for free, so you must also check this out also. You may also need to re-install Java since it removed the previous version after the upgrade. It is available in Apple's website for you to download here: http://support.apple.com/kb/dl1572.
Then, to see all of the added features in Yosemite, you can check it out here: http://www.apple.com/osx/all-features/.
