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Mail now gives you two and three column views and it has features like a Web browser with a favorites menu on top.
Another feature is AirDrop that lets you see other people on your network just like you would with computers. Then you can drag files to them.
You can also see groups of information to follow the flow of a conversation.
And it's $100 cheaper than previous OS X upgrades. Plus, you can only get it through the Apple AppStore.