This makes the situation worse does it not ? Now NXT owners will lease their coins to a handful of operators to earn fees.
These operators would be perfectly placed to mount an attack. They would have a % of the POS coins and they can mount a double spend attack with pretty much zero risk as identified by DaT.
It's similar to the situation with Bitcoin hashing pools. A few hashing pools together control over 51% of the hashing power, so that becomes a vulnerability for Bitcoin. The difference is that it's easier to set up a forging pool than a hashing pool, so hopefully we will have more of them, and the power will be less centralised. But yes, leased forging makes it feasible to gain a large fraction of forging power for little financial outlay, and that's something the community needs to be vigilant about (much as how the Bitcoin needs to be vigilant about hashing pools).
(Nxt has the additional danger that forging power is effectively moved around by transactions, and the node that forges a block gets to choose which transactions they include, so they could reject transactions that transfer forging power away from themselves. That is why forging leases are temporary, so they will eventually expire even if no transactions get processed. I'm not sure that's enough. In this regard a hashing pool is more responsive, because nothing can stop an individual hasher from withdrawing from a rogue pool immediately.)
I don't think most of DaT's comments apply to Nxt, because it's such a different algorithm not based on coin-days destroyed. For example, it doesn't use checkpoints so all the discussion of those is irrelevant. Unfortunately I don't understand Nxt's algorithm too well myself; and even if I did, they plan to change it to something called "transparent forging" in a few months, and the details of that are being kept secret for fear of clones.
Doesnt matter what you call it ... coin days destroyed, stake lending or transparent forging. The fact is that stake holders who owned and sold their stake they can repeat the same method and produce an alternate chain of the PoS coin and claim their stake back by showing more stake power. Old stake owners will always have an incentive to do that because of profit.
Add to the fact that increased economic activity will reduce the stake used for minting (which is already very low for PoS coins) and thus lower % of stake used to secure current chain it is a recipe for long-term disaster or eventual centralization. From central bankers to central programmers.