Thanks for the info, two more questions:

• Is NXT's Transparent Forging vulnerable to attacks by old private keys in the way that normal PoS is? Or does one have to control 51% of the current forging stake in order to attack it? If this is not vulnerable, then getting control of 1% of the stake seems as hard as controlling 51% of bitcoin hash power, and the person controlling this stake would not wish to harm their own stake. (The risk in the attack is if one can sell coins and then still attack).

• Is Bitshares' DPoS vulnerable to such attacks? It seems that in order to attack the network you would need to compromise 51 out of 101 delegates, who produce the blocks. The delegates are voted on by shareholders, and you can vote again at any time. So if someone held stake, used it to vote in delegates that would attack the network, and sold their stake, the new buyer could vote in different delegates right away (if they were paying attention).

I suppose that you could social engineer your way into a position to attack by building a reputation and then getting 51 delegates elected without people realizing they were all part of the attacking group. Is it harder to fool everyone, or to get 51% of mining hash power?

As long as these systems are at least 'almost' as hard to attack as bitcoin, then they seem useful as a low cost alternative for applications where absolute security is not required. That is, maybe you arent trying to be the ultimate store of value like Bitcoin is. For many applications, high security at a low cost would be a better choice than perfect security at a high cost.