Steemit is not a Ponzi scheme as it is run on the STEEM currency therefore that value can be used external of Steemit at anytime. If you don't want to use STEEM as STEEM Power you can just own STEEM in a wallet or another exchange.

Commenting as far as I can tell is rated by voting power. So if you have more voting power your comments will remain near the top. It helps to filter comments by reputable users.