Comments were one of the more basic features in Drupal 7. There was only type of comment and you had very limited modification and moderation options.
In Drupal 8, comments are vastly better. Here are 4 ways that comments have changed and improved:
#1. Comments are fields
In Drupal 8, you add comments as fields. This allows you to be more flexible with where comments are displayed on a page. It also allows comments to go anywhere that fields are allowed (not just on content).
#2. Comments have types
In Drupal 8, you can have more than one type of comment.
For example, if you run a social networking site, you might want to allow admins to leave private notes on users:
As I mentioned above, Drupal 8 allows you add comments to more than just nodes. You can add comments to blocks, contact messages, content, taxonomy terms, users and even comments. Yes, you can add comments to comments.
However, without the user of contributed module, there may be a limit to far you can take the example I've just shown. This is because Drupal 8's comment types don't have individual permissions. Anyone with persmission to post on type of comment will have access to post all types. Again, you can expect a contributed module or Drupal 8 update to fix this.
#3. Comments have fields
Not only are comments now fields in Drupal 8 (see change #2 above), but you can also add fields to comments:
#4. Recent comments is in Views
As with many things in Drupal 8, the Recent comments block has moved to Views. In Drupal 7, the Recent comments block couldn't be edited.
In Drupal 8, Recent comments is available by default: