Handoff, one of OS X Yosemite's standout features, makes it super simple to create a deeper relationship between your Apple devices. Working on a project on your Mac? You can pick up right where you left off on your iPad, and vice versa. For the sake of productivity, it's a really convenient feature, and means you can get work done no matter which device is available to you at that moment. Unfortunately, it won't be quite as accessible to everyone, depending on the devices you own.
As first noted by Apfeleimer, the only way users can take advantage of the continuity/Handoff feature is if they have devices with Bluetooth 4.0 LE support. For most users of newer devices that shouldn't be a problem, but going back to the 2011 MacBook Air, Mac Mini and before, the feature likely won't be supported. Besides that, anything before the iPhone 4s and iPad 2 won't be compatible with iOS 8, so it might finally be time for you to upgrade; the iPad 2 won't support continuity because it lacks Bluetooth 4.0 LE support, so bear that in mind.
Apple hasn't yet confirmed which of its machines will support what is possibly Yosemite's best feature, and there's nothing available on the company's site just yet. For power users, the lack of continuity will be sorely missed, but I imagine many of these users will be upgrading soon anyway. There is, after all, reports of a MacBook Air with Retina screen coming later this year.