Any user can comment on ideas, via the expanded idea view.
To post a new comment, type a comment in the box and hit the 'return' button on your keyboard. Once this is done, your comment will drop into the activity timeline below the idea:
It is possible to 'like' other people's comments within the expanded idea view, by clicking 'like'. You can also filter the interactions, to just show comments, ratings and favourites too:
The total number of comments attached to an idea is displayed on the idea card view too:
You can also tag other users in comments.