2) other coins do have multisig, but not blockchain based multisig. there is a MASSIVE difference.
Really? Then what is this?
https://en.bitcoin.it/wiki/Address#Multi-signature_addresses
I am not the best to answer this, but I will give it a try. With Bitcoin, each service has their own multi-sig adaption; that they have to run extra software that they design themselves on top of the regular Bitcoin client. Coinbase does it there way, GreenAddress does it there way, and Bitpay is doing it their way, and so on. As far as I know, I can't just download the blockchain and run a Bitcoin client on my computer with a general multisig feature all by myself. So if I want multisig in Bitcoin, it means teaming up with a third party (as far as I have seen, please correct me if I am wrong). Further more, if I have my coins in any of these services and they go down, my coins are stuck. For this reason each of them offer open source support software on Github but its not the type of thing a regular person would be able to understand or use. It'll take a real techie person. You could probably do it for instance, but I couldn't.
At NEM it is different. Any third party can build on NEM and build on a multisig feature that is shared by all. I can make a multisig account on my computer all by myself with no help from a third party, and then I can throw my computer away and disappear in a cave. But if you are my cosigner on that account, you can now initiate and send from that account no problem as long as you get the other cosigner(s) to sign.
So all of a sudden multisig accounts can be made here, and operated there, and even multiple third party apps built on NEM can all be a party to the same multisig transactions. Say for instance you use NEM asset exchange webpage to make a multisig account. I can sign a transaction on my mobile app made by my friend, and somebody else can sign it on the desktop app made by the core devs, and so on.