what exactly distinguishes a Ponzi-type game from other gambling?
Gambles have provably fair rules.
Ponzis are scams in which, by design, the author can pull the plug whenever he sees fit.
In the best interest of young people and/or newcomers who might not understand this I would add a new subforum called "HYIP/Ponzi/scams", or directly "Scams".
On the other hand you can never guarantee 100% of the gambling
sites out there are provably fair by any means, not even 10% of
them. Besides they can also refuse withdrawals whenever they see fit.