What does it have a "stake or die?" poison pill?
Something that would "negatively reward" hoarders?
Like the "expiry" built into Namecoin
No actually not. The stakeholders are the fortune of Nxt because they protect and ensure further development of Nxt just because they want their share to become more and more valuable.
After receiving one NXT from another account, that NXT takes an idle period of 1440 blocks. After that time, that NXT simulates a small mining rig. And as you know the more mining rigs you have, the more blocks you mine. Same here except that we call it forging AND that the process is a deterministic pseudo-random process.
Pseudo-random because at each block the queue of forgers that are going to forge are pseudo-randomly shuffled (with maintaining the probability: the more NXTs the higher the probability that you forge)
Deterministic because we can predict the next forgers and can therefore build a system called Transparent Forging that allows us to minimize network load. That is required to get Visa-like transactions per second.