As mentioned before, the comment system of Google+ is very similar with the one of Facebook, you can "+" a post or its comment, you can share a post, and you can also reply to a post as well as its comments.
But there are some differences between Google+ and Facebook on how to reply to messages (including posts and their comments).
On Facebook, you can reply to a friend’s message with "@name" in the beginning of your answer, something like what you need to do when mentioning a friend on Twitter; and on Google+, you can reply to a friend’s message with "+name", such as "+Young Yang".
As you can see that the reply process of Google+ is as easy as that of Facebook, but not easy enough, so someone has created a tool to make it easier.
As a free Chrome extension, Replies and more for Google+ is such a tool for you to reply to your Google+ friends’ posts and comments in one click.
After installation, you will see a "Reply to Author" link near to the "Share" link under every Google+ post, clicking the link, you can answer to the author. And hovering the cursor over every comment of the post, you can also see a "Reply" link near the "+" button, clicking the link, you can reply to the comment author.
By the way, you can reply to multiple friends at a time, just as what you can see from the above image.