I believe in the Quaker rule, "Only speak when you can improve the silence." Other people think speech is like squatting on land. You have to speak to gain footing. By charging people for the privilege of speaking, you make them consciously decide whether what they have to say is worth the $5 to join. They will probably say no.
* posting a comment
* voting on a comment
* submitting a link / question
* voting on a submission
Each of those are debatable. For instance, submission voting might be open to all but comment voting is not.An interesting variant: You can 'keep' a link without paying, but if you are paying, your 'keep' is the counted as a 'vote'.