When you find a good thread in Google+ and want to keep abreast of what people are saying, you need to have your email notifications set to “on”. That way, whenever someone replies to the post, you’ll get an email keeping you up to date on what people are saying about that item. Google has just improved Google+ so that when you get that email, you can reply right back to any comments directly from Gmail.
One of the cooler elements of this integration is that when you get the Google+ notification in your Gmail and look at the post, Google has enabled it so that the layout is as if you were looking at the Google+ post directly, and the comment button is right there in your email. Take a look at the screenshot below.
This is different than replying to a Facebook message post notification, for instance, where you have to reply to the email in plain text and hope that it will send. The fact that Google+ actually shows the post and the “post comment” button is very satisfying and helps the user understand that this is the same as posting a comment on the post on Google+ itself — Google is just saving me from having to click over to the post itself.
Google also announced that users will be able to reply to posts from their mobile phone, by simply responding to the email itself. This is similar to how Facebook does things at this point, but is a useful feature nonetheless.