Adding comments to a list item

it might not be exactly what you had in mind, but you could always just use a Multiple lines of text field with 'Append Changes to Existing Text' on. Version history will keep it so that you can see who added what comment. This is the simplest way to accomplish this. This of course assumes that permissions are not restricted on individual list items.

We have implemented same concept for our internal Idea Board solution. We have a multi line textbox field to store the comments in JSON object format. The comment object will take "Date","User", "comment". This is a custom solution using JSOM.

I can help you further if you are looking same kind of solution.

Thanks, Venkat

There is a very nice JavaScript-only solution from spjsblog.com that you could give a try: http://spjsblog.com/2014/03/04/commentbox-for-sharepoint-2010-and-2013/

There are different ways to do it in SharePoint (Append-Only-Comments) in the out of the box columns which keeps the old comments too. follow below link for more understanding https://chuvash.eu/2012/04/02/add-comments-column-to-your-sharepoint-list/enter link description here

Also you can achieve this by using CSR in SharePOint list

http://letsdosharepoint.com/2015/08/13/how-to-add-read-more-link-in-sharepoint-list-using-csr/

There is another way using call out function in sharepoint

http://www.learningsharepoint.com/2013/05/09/sharepoint-2013-callout-popup-tutorial/

http://social.technet.microsoft.com/wiki/contents/articles/24058.sharepoint-2013-callout.aspx

You can embed an open graph Yammer thread into the item detail page; this is a cool solution that means they don't have to go 'edit' every time users want to comment on a list item; or indeed have to edit the item.

TL;DR
short answer: you can't easily add comments to lists other than discussion boards.