I'm a twelve and a half year old girl. I truly understand why parents may not give a tween-going-on-teen's review, but please give me the benefit of the doubt. Here is what I think of Google +:
1.) Like any social media site, be sure to check in on their account, and explain to your child (ren) the importance of not sharing any personal information: phone numbers, email addresses, home address, etc.
Also be aware that strangers can communicate with your child through Google + by using Google Hangouts. While chatting requires an invitation by default, set boundaries with your child and regularly moderate this if your child (ren) frequently utilize this feature.
Furthermore, if strangers add your child to their 'circles' (follow them), they can see any post your child makes. However, your child can choose to make the post either public or private. Talk to them about this.
2) About communities: this is something that must watched closely. While it is usually quite appropriate (95-98% of the time), there is always that possibility of abuse. However, inappropriate communities usually require you to send an invitation to be approved.
To learn more, search 'google communities'.
3) In general, there isn't a lot of swearing, as far as I could tell.
Just use the same precautions, such as informing them that 99% of what they do on the internet is traceable and that once you post/comment/send something, it can't be taken back, and it will be smooth sailing.
Quoting a 13 year old reviewer above me, "It's not the site that's bad, it's what you search on it."
Google + has many more privacy tweaks in place, but be careful.
This site is alright for 12-13 and up.