While everybody's been praising Google+ over its beautiful interface design, cute animations, and utterly reasonable privacy model, I've found myself not using it. I can think of two reasons for that - one, it's still bound to my ancient gmail account and not the Google Apps account I currently use, and two, it makes me think too much.
Making me think too much about my social networks could be a real blocker. In forcing every one of your connections to be classified into a specific group ("professional contacts", "friends", "bdsm buddies"), Google+ really makes you think about how well you know this contact, what you might want to share with them, and where the best place to file them is. Personally, I have a lot of overlap between multiple groups of friends, coworkers, and professional contacts.
The problem gets compounded when you have to decide who to share each item with. Since you have to choose who you want to share things with on every post, you have to think a lot about your circles and decide who would be interested in what, and who you're comfortable sharing things with. Or just mark everything 'public', in which case Google+ isn't much better than a public blog.
Finally, the problem in deciding who would be interested in a post I want to share is that I'm never sure who would be interested in what. When I post something to my tumblr / twitter / facebook, I'm usually surprised by who responds. If I post some code I wrote for a server monitoring script, I can guess who's going to be interested in that. But if I post some video game news I'm excited about or a music video I like, my coworkers are just as likely to be interested as my friends.
With Twitter, everything is public by default - you don't have to wonder if it's okay to post that racy dream you had about your boss last night. You know it's not. If you make your account private, whenever you allow someone to follow you you have to decide if you want to allow them into your private life, but it's a binary decision. Yes or no. It's not complicated. Facebook is the same - either you let someone in or you don't. But Google+ encourages you to think harder, and personally I don't want to put that much effort into social networking.