Interactive previews allow you to hover over someone's username to get a preview of their profile, including content that they've recently posted. When you are logged in to the community, you can send a private message, follow a member, and request friendship directly from the hovercard.
When you use the @ symbol, you now have more options. Not only can you What is @Mention people and content, but you can now @mention groups in the community. The video below is an example of how you can @mention content and groups in the community.