I'm by no means an expert but from what I understand, there proof of importance concept is to prevent double spends which has been an issue with quite a few altcoins.
The immediate issue I have with NEM is the whole idea of vested balance. In such a volatile market old money is dangerous. There are other altcoins such as Siacoin and BitShares with much more impressive tech.
Vesting is almost fully complete after two weeks and you only need 10,000 XEM to harvest. The vesting period is another security mechanism that makes Sybil attacks more difficult.