By default, all users (including anonymous ones) can add new comments, but this is naturally customizable.
<comments />— basic comment form, sufficient for most things
<comments>Allow="Derfel,Jack Phoenix,Misza"</comments>— only allows the users Derfel, Jack Phoenix and Misza to comment on the article.
<comments>Voting="Plus"</comments>— prevents users from rating comments negatively (thumb down), allowing only positive (thumb up) votes
Ignoring other users' comments
You can also ignore comments from other users. Click on the little "block" icon next to the user's name and user level. You will then be presented with a confirmation message that asks if you really want to ignore all comments from the given user. If you press "OK", comments from the given user will be ignored and his/her user name will show up on Special:CommentIgnoreList.
Comments from ignored users will be hidden, but you can click "Show Comment" on a hidden comment to view it.
By default, comment-related actions are not logged. If you want to enable comment logging on your wiki, contact a member of the Customer Support Team.