After reading this article it seems to me that Steem is a ponzi-scheme according to dan's own words:
It only becomes a Ponzi scheme once NuBits is forced to print new NuBits to pay interest on old NuBits
The NuShareholders vote on an interest rate that will be paid to NuBits holders whom volunteer to park their NuBits for a defined period of time. The interest is paid by the issuance of new NuBits. In other words, parked NuBits get a return paid from future investors. If there are no future investors then the interest rate would go to infinity and the entire dollar peg would break down. This is the classic, text-book, definition of a Ponzi Scheme.
Don't shoot me for bringing this up :-)