From new Continuity features, to smaller useful tweaks across all the core apps, Mac OS X Yosemite is a must download for Mac users.
When you compare icons between Mavericks (top) and Yosemite, you can see the flatter design that looks a bit like iOS 7 while still remaining unique to the Mac.
With Continuity features, you can have calls to your iPhone forwarded to your Mac. You also have call waiting, and can quickly add contacts to a conference call.
New Handoff features let you work on something on one device, then quickly pick it up on another.
In this slide you can see the Notes icon to the left of the Finder icon indicating I was working on notes on another device. One click, and I'm right where I left off.
In iTunes, there aren't a lot of changes to the store itself, but navigating your content is much easier.
You can use the icons in the upper right to quickly switch between music, movies, and TV shows, and a drop-down menu on the right reveals the rest of your content.
In Notifications, the new Today View lets you see what you have on tap for the day. You also can add widgets by using the Edit button at the bottom.
In Safari a new Tab View lets you see all your open tabs so you can quickly switch to the correct one.
When you perform a search in Spotlight, now you'll also get Web results. A search for "Chinese" gave me a few restaurant locations nearby, and the phone number, address, and directions (using Maps) are all only a click away.
If you scroll down in spotlight, you can see photos from the restaurant you selected and view Yelp reviews below that.
We could already use AirDrop on iOS devices, but now you can AirDrop files between your iOS 8 device and your Mac.
In Messages you'll be able to start a conversation, then add more contacts to make a group chat. If the notifications get too intense, you can always select Do Not Disturb or remove yourself from the conversation entirely.
With Mail Markup, you can attach an image or document and then add arrows, text or other graphics to point out or explain certain parts.