New OS X features exist in a spectrum. The OS change can range from a under the hood improvements to UI cleanup to improving functionality to an outright new feature. Sometimes the feature is minor, and sometimes it's a blockbuster.
For example, OS X 10.9 Mavericks brough us App Nap and Compressed memory under the hood. Contacts and iCal brought a new look and feel. Multiple Displays tweaked and improved the functionality. Calendar brought integration to Maps. Finally, Apple brought Maps and Tags to the OS.
Sometimes, the improvements just sort of glide by without too much focus, and they're there when you need them or go look for a solution. They're so obscure, they need a TMO Quick Tip. However, sometimes a new feature becomes the signature feature of the release.
Yosemite's Major Features
Here's a look at the interesting features of OS X 10.10 Yosemite. I'll grade the features from F to A based how useful I think they'll be (for me anyway) and how likely it is that it will become a signature feature. [The list is in no particular order.]
1. Phone calls (A). I think this will be a game changer for several reasons. My enthusiasm is based around the idea that when my iPhone rings, and it looks like I'm going to be on the phone for awhile, I ask the caller for a second to attach a headphone. Alternatively, the sense of separation and audio quality of the speakerphone isn't so great. Using the first class Yeti microphone and speakers of my Mac is the way to go.
Plus, when I'm in my recliner with the MacBook Air writing, and my iPhone is in another room, charging, I won't have to kick my cat off my legs, my MBA off my lap and go running. It'll be awesome.
2. SMS messages (C). I am not a huge user of SMS. I do use iMessage occasionally with other Apple friends, but I figure an SMS message from a non-iPhone can wait until I get to my iPhone. When I'm writing on my Mac, which is most of the time, I try to avoid distractions anyway.
And that brings up larger issue. When I'm working, I hate distractions. I have "Do Not Disturb" turned on for Notifications during writing hours. I do listen to iTunes for mood music and monitor our TMO chat room, but that's about it. And so I see SMS messages on the Mac as a pleasant and distracting tool for some, an annoyance for others.
3. Handoff (C). Having said all the above, you can probably imagine how I feel about starting an email on a Mac and continuing it on an iOS-device. I think that's just a sample use-case, and the real advantages will evolve and come to light later. It doesn't light me up.
Next: Features #4 Through #8.