The company said more than 170m people have "upgraded" to the social network so far.
But critics say the number of regular visitors is much lower.
In a blog post, Google's Vic Gundotra wrote: "It's still early days, and there's plenty left to do, but we're more excited than ever to build a seamless social experience, all across Google."
He said the new features included "full bleed" media - showing pictures and video in full-size and quality - and "conversation 'cards'", which would make it easier to have threaded conversations, similar to the format found in Google's email service, Gmail.
Stand-out features The service now displays a list of the most commonly discussed subjects, known as trending topics. This feature mirrors microblogging site Twitter, which first introduced trending-topic lists on its site in 2010.
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David Philips Social media expertYou can use many more of the features more intuitively”
Also borrowed - from Facebook - is the cover-photo feature - a large image displayed behind a user's profile picture.
Cover photos were introduced to Facebook profiles as part of
its timeline redesign and rolled out to its users over the past few
months.David Philips, a social-media and PR lecturer, told the BBC he believed the revamp had made it easier for Google+ users to use the network's stand-out features.
"It opens up the architecture of Google Plus so you can use many more of the features more intuitively," he said.
"I think it's now becoming a serious small-business tool, and also a very interesting tool for families where they can have lots of information shared among members of the family."
He added that while Google+ might struggle to stand out among its bigger, more active rivals, the breadth of integrated services would soon begin to work in the network's favour.
"I think this is a development that helps it carve a niche because it allows people to use so many different tools at the same time," he said.