So, apart from the guys shouting "rich will get richer"
It's not that. It's the rich will have untouchable anonymous power. There is no way to stop a cabal of people from taking permanent control and reversing transactions discretely.
But with PoS they need 51% of the entire wealth of the currency, to do so, and at that point why would they do it? they will only undermine themselves when discovered because the value of the currency will plummet.
They would never allow themselves to be discovered. There is no method of tracing controlling shares to individuals. Everything would be done in secrecy using TOR or other darknets and disinformation campaigns would be waged.
With PoW, they only need about 10% of the wealth to overwhelm the mining hashrate, because the current Bitcoin mining operations are worth about 10% of the Bitcoin marketcap.
There are many countermeasures against PoW attacks precisely because the hashrates are known. There are still vulnerabilities with Bitcoin, but because the transactions are more transparent, there will be better engineering solutions.
What are you talking about "they will never allow themselves to be discovered"? I can assure you, they will be discovered once they start reversing transactions. The identity doesn't matter. This particular crypto-currency will crash and burn over night, the attackers will be only hurting themselves since they hold over 51% of the currency.
On the other hand, attacking a PoW currency only need about 10% of the marketcap in cost. How does "hash rate is known" make it any harder to attack?? I don't understand, I would think it only makes it easier to plan the attack. In a PoS currency, the "unknown" part is really a safety mechanism, since the attacker can't easily figure out if he could ever obtain 51%, because he don't know who's holding what, and 51% may never be achieved simply because they are not available for sale.
with nxt you would need to hold 91% of the coins to do a "91% attack" against the network