Some people don't like change, but some of the improvements in OS X 10.10 are for the better — especially if you use it constantly, or over long periods of time.
Spotlight Search now appears in the dead-center of the screen you're working on, which is particularly handy if you have very large displays or multiple screens. It overlays what you are working on, and can pick out everything from suggested websites, your locally stored apps and data, to system preferences, and other items from the App Store. It also offers a better preview of what you're looking at so you can easily determine what you want.
Even running third-party apps, such as Microsoft Outlook for Mac, you can filter through emails and other non-Apple app data. With text documents and images, you also see a preview of the file so you can be sure that's exactly what you're looking for.
The new Messages application has a new user interface and experience, exactly like the Finder app. You can now also send traditional text messages (SMS) through your iPhone and read them on your Messages app, keeping even non-data iMessages in sync with your Mac, so you're never out of the loop.
If you need to drop someone a quick call, you can do this from Messages via video-calling or audio-only. And if things get a little noisy? You can simply hold back any notifications from that conversation.