The problem with this I feel and one of the advantages of Twitter is that if a certain company controls their community page (e.g. Audi USA) and I go to post some sort of customer service complaint, they have the option of removing it. With Twitter, I can just add a hashtag or @message them and it'd be visible globally with search.
wait, so what prevents you from +mentioning someone or #tagging a post in G+? Communities are completely independent and orthogonal to those already existing features
