To let someone who is signed in be able to comment on a card, go to it's rules, click on the menu to the left of "...can comment on this card", select "Anyone signed in" and click "Save Changes".

There's nothing coded in to Wagn that makes cards ending in "+discussion" have comment boxes. We configured new Wagns to treat names ending in "+discussion" this way by using Wagn's structure feature. We built in a card called discussion+*right+*default and set it so that "Anyone" has comment permissions. Because of that formatting card, whenever a new card is created with a name ending in "+discussion", it will have commenting turned on by default.  But you could turn that off (for new cards ending in "+discussion") by changing the permissions on discussion+*right+*default. And you could have comment boxes on, say, new cards that end with "+talk" by creating "talk+*right+*default" and setting it so that "Anyone" has comment permissions.