Stop thinking like an engineer and think like a non-technical common user.
Being a non-technical common user doesn't mean you don't need better filters; it means the service needs to make it easier for you to explain to it what you do and don't want to see. You don't even have to expose the word "filter" to the non-technical people, but the filters need to be there conceptually, nonetheless.
Note that I'm not saying this is easy... I'm saying this is the challenge that G+, Facebook, Twitter, etc. need to take on and solve. As opposed to the challenge of "how do we not let people post stuff." That's easy, and it's a cop-out.
I think the real challenge for Google would be to make it easy for the user. I have no doubt they could build some awesome filtering logic in the back. But I'm not sure they could put a super easy, non-engineer style front on it. But I agree they should try. And who knows? maybe they are working on that and want to get it rock solid before they open the flood gate. Build the filters before everyone screams "we need filters"
You can try to make it easier (probably a UX challenge) but most people don't take the time to be meticulous with setting up their filters. e.g. Gmail filters, Twitter lists, etc.
In the case of Twitter, people actually might if the interface was better. People did, before the interface changed.
Now, adding a large group of people to a Twitter list via web Twitter is such a painful exercise, I often turn to an API solution (tweetbe.at in particular) to do it.
Being a non-technical common user doesn't mean you don't need better filters; it means the service needs to make it easier for you to explain to it what you do and don't want to see. You don't even have to expose the word "filter" to the non-technical people, but the filters need to be there conceptually, nonetheless.
Note that I'm not saying this is easy... I'm saying this is the challenge that G+, Facebook, Twitter, etc. need to take on and solve. As opposed to the challenge of "how do we not let people post stuff." That's easy, and it's a cop-out.