1. There's a thrill of the new thing going on. You can start over with your friend lists. Start afresh. And early adopters love new toys.
2. They do photos really well. Sharing with Picasa is really good fun. All my photos are in Picasa. Now Picasa one-click defaults sharing to Google+
3. Google does APIs and tech stuff really well. So I presume that the tools they will make available through Google Labs will be extensive, fun, and flexible.
4. They have a HUGE graph. Imagine the users of YouTube, Picasa, GMail bringing it all together? I spend more time in Google apps than I do in Facebook. Facebook feels like I'm in a separate state. Google+ feels like another little town in my conurbation.
5. Sharing by Circles makes sense. It's what Facebook Groups should have been. But the interface stunk. I like the little animation of dropping Guy's face into different circles. It's fun, and it's really obvious and easy.
6. Google+ seems to have cleverly hit upon a Twitter/stream idea with following etc., and Facebook private sharing. People are "following" me, but I don't have to share with them if I don't want to. They will just see what I make public for them. It's like Twitter is built in. That's powerful, and probably the most important point, so should have been #1.