Change permissions for new users to comment
34339
Lower the amount of reputation needed to comment
95252
Could new users see a notification in the space where the add comment button would be, like "you need a minimum of X reputation to comment"?
92948
Allow Comments on Deleted Answers
11036
Changed permissions of / to user
23614
A method for the OP to accept comments?
142814
How do you remove the ability for a user to make a comment or post on a page?
147476
Do permissions apply across users?
105916
Auto-expand @username in comments to keep the username up-to-date
76476
Change all @username references in comments when a user changes their name?
60713
Giving a single user write permissions
23993
Allow moderators to edit/delete comments directly from the per-user comment moderation page
129908
Allow flagging own comments
120358
Show the "add comment" link also to users who cannot add comments
87776
Allow voting on comments from Review interface
8537
Could we please be a bit nicer to new users?
123485
How can I limit other (administrator) users access to my profile?
144201
Can I place  in a comment?