The first thing that strikes me is the UI. I think Google+ has a fantastic user interface. It's simple, clear, and easy to learn. One thing that I really like is Google's attention to details in their user interfaces. Whenever you click "+1", there's a little animation of the number rolling up. If you delete a "circle", there's a little animation of it rolling away off the screen. These little things contribute to a great user interface.
Compared to the Facebook UI, Google+'s UI is a breath of fresh air. However, Google+ only has a tiny (really tiny) subset of Facebook's features. This probably contributes heavily to Google+'s simple UI. I suspect that when (if?) Google+ gets all the features that Facebook has, the user interface will become a lot more cluttered. With that said, it's not hard to beat Facebook's user interface.
This sort of leads me to one Google+'s biggest drawbacks. They really offer a very limited subset of Facebook's features. There are no events, messages, chat(EDIT: Just kidding. They have chat), or even "wall-to-wall" posts. An application API is also missing (Farmville+!). Granted Google+ is still at a very early stage, so it might get a lot of those features later.
The other big drawback is userbase. It is very hard to have a successful social networking application without a lot of users. People won't switch to Google+ until their friends switch. Of course, their friends are thinking the same thing. I think Google can overcome this problem fairly easily though. Perhaps we'll see migration tools that let you quickly populate your Google+ account using your Facebook data.
There are a few neat features in Google+. The one that impresses me the most is the idea of Circles. With Circles, Google+ lets you place your "friends" into various groups. Then you can choose which groups, or circles, can see what content. This is a nice way to keep your family from seeing your status updates about drinking and partying.
Another benefit is that Google has a much more sensible TOS than Facebook. They also have a better history of protecting things like privacy. I know for a lot of people, this is a very big deal. I personally don't care too much about this one. When you put things on the internet (especially on a social networking site), you always risk that everyone might be able to read it. This is why I never post things like my phone number on Facebook (even if its just for "Friends"). The only information I have on Facebook is information that I would feel comfortable telling strangers.
A huge problem I've had with Facebook is their rollout strategy. They seem to be fans of release early, release often, but they suck at it. It is almost a weekly occurrence when a major piece of functionality is broken. Facebook doesn't take enough time to do regression testing before they push updates and it really bugs me. Just because you can fix it fast doesn't mean you can ship it in a broken state. >_< I've found Google to be much better in this area. They also progressively add to their software, but it isn't crippled every week by stupid release strategies.
I'll keep an eye on Google+ going forward, but they have a lot to do before they can realistically hope to beat out Facebook.