[vbuterin]

Altruism and defaults are powerful things. In fact, they're the only thing enforcing rules about "nonstandard transactions", without which the blockchain would have been DDoSed all the way to a terabyte by now.

And the checkpointing system is a backup measure used to maintain stability because PPCoin is still small; it is not intended to be the primary mechanism holding it up. If you want to see an altcoin without checkpoints, look at any of the ones that got killed by a 51% attack.