It abandons Facebook's "one-message-fits-all" approach to social networking
Facebook treats all of your "friends" alike. With it, you essentially have the same online relationship with someone you've absent-mindedly accepted as a friend, but never met, as you do with your spouse or friend of 15 years.
Not so with Google+. With Google+, you create separate "circles" for your online acquaintances, and then post separate messages to each circle. Want to make plans the July 4 weekend with your five best friends? Then post a message just for them. Phony Facebook "friends" won't see them.
It sparks conversations
One of Google+'s niftier features is Sparks, which combines the power of Google search with social networking. Powered by Google search, Sparks delivers a stream of blog posts, news, and videos about any topic in which you're interested. When you want to share any of that with others, click the Share link, and you can share it with your circles.
It integrates with other Google services
If you use multiple Google services, you'll especially welcome Google+. Conversations in it show up in Gmail. You can share photos from Picasa using it. It has direct links to Google Talk. Expect more links as well.
It's better for mobile
Unlike Facebook, Google+ was built from the ground up with mobile in mind. So it works far better with your smartphone. (There's an Android app for it now, soon to be followed by one for iOS, according to Google.) You can do group texting via the "Huddle" feature, for example. Uploading photos and geotagging is a snap. You can get text messages whenever there's new activity on Google+. While it's true that Facebook can do some of these things, it simply doesn't do them as well as does Google+.
If you want to read my full review of Google+, check out First look: Google+ lets you hang with a small (or large) circle of friends.