Added the following to the post on the first page of this thread that enumerates the flaws of PoS (proof-of-stake).
Even smooth had mentioned (in my vaporcoin thread) that PoS devolves to effectively PoW in order to defeat the nothing-at-stake flaw.
Following is written by
David Mazières a PhD professor at Stanford who is the
Chief Scientist at Stellar.
An alternative to proof of work is proof of stake [King and Nadal 2012], in which
consensus depends on parties that have posted collateral. Like proof of work, rewards
encourage rational participants to obey the protocol; some designs additionally penal-
ize bad behavior [Buterin 2014; Davarpanah et al. 2015]. Proof of stake opens the pos-
sibility of so-called nothing at stake attacks, in which parties that previously posted
collateral but later cashed it in and spent the money can go back and rewrite history
from a point where they still had stake. To mitigate such attacks, systems effectively
combine proof of stake with proof of workscaling down the required work in pro-
portion to stakeor delay refunding collateral long enough for some other (sometimes
informal) consensus mechanism to establish an irreversible checkpoint.