The upgraded commenting system preserves the existing comments, but the future comments require a Google+ account. That means, visitors can no longer post comments anonymously, using an OpenID account or using a Google account that hasn't been upgraded to Google+. When posting a comment, visitors can also share it on Google+. The new commenting system doesn't just show the comments posted on Blogger, it also shows all the Google+ messages that link to the post and their comments.
Just like Disqus, Google+ Comments shows by default the best comments and there's an option to show the most recent comments. It's not clear how Google determines the top comments, but the number of +1s could be one of the signals. You can also restrict the comments to the people from your Google+ circles.
Bloggers will notice some other changes: the comment management feature from Blogger is no longer available and you can only manage comments for each post. That's a significant downside, especially if you receive a lot of comments. When one of your posts receive new comments, you'll see a notification in the Google bar and you'll get an email notification. I haven't found a way to disable these notifications, not even from the Google+ settings page.
How to enable Google+ Comments if you have a Blogger blog? "Google+ Comments is available for Blogger users who have created a Google+ profile and connected it with their blog. If you haven't upgraded your Blogger profile to a Google+ profile and would like to, you can start the process by clicking the Google+ tab on the Blogger dashboard. Once your blogs are linked to a Google+ profile, a new 'Use Google+ Comments' setting will be displayed on the Google+ tab of your blog," explains Google. If your blog has a heavily modified template, you may need to reset widgets or use the code from this page.
There are additional limitations: Google+ Comments is not available for private blogs and for blogs with adult content. This means that the old commenting system will continue to be available as long as Blogger supports these kinds of blogs. You can always disable Google+ Comments and go back to the legacy Blogger commenting system.
It's important to point out that Google+ Comments are embedded as an iframe (the URL starts with https://plusone.google.com/_/widget/render/comments) and the comments are no longer included inside the Blogger page. Google manages to index content from iframes, but the change could affect your Google rankings.
Another interesting thing: it's the first time when the official Google Blog has comments.
Some early thoughts:
* Blogger is now more relevant, it's less likely that it will be discontinued in the next few years
* Blogger finally has a modern commenting system
* Google+ Comments will be available for other blogging platforms
* "sign in with Google+" will make Google+ even more popular as an identity service.