To use the comments feature, install JComments component (www.joomlatune.com). Select and enable comments on the general configuration.
Plugin file and a new icon on your dashboard, will automatically be installed.
Then you will be able to define if you allow comments on specific types, categories or items in their parameters
In version v1.5.6, comments in item view have the expected intuitive behavior: they inherit parameter setting from the current category.
So if you enable comments in category then the items of the category will have comments enabled