This tip arrived with iOS 8 and Yosemite, but is a little down in the weeds, so you may not have noticed it.
First things first, if you want to share tabs between your Yosemite powered Mac and an iOS device running iOS 8, go to Settings > iCloud and make sure the Safari switch is on.
Now go to Safari on your Mac and click the “Show all tabs” button (the icon with two squares to the right on the window’s title bar). You should see all the current tabs, plus sections for each of your currently shared iOS devices. You can tap one of those tabs and it will open. Tap the little x to the right of a tab and it will be closed on the iOS device.
On the flip side, go to your iOS device and launch Safari. Tap the “Show all tabs” button (lower right) and you’ll see your carousel of tabs. Slide that group up and you’ll see the list of open tabs on your Yosemite machine, as well as any other iOS devices. See a tab you want to close? Slide it to the left and tap Delete.
This clever detail is exactly the sort of thing that draws me to Apple products.