I think it's a good idea but I don't think they're are going to be able to put in place a system that prevents someone from buying a drone until they register it. Plus, many people build their own. I believe what's going to happen is you're going to buy a drone and when you open the box you're going to see an insert that says you're required by DOT to register your drone and here's the URL. From there it's up to you whether you register or not and then the burden is put on our local law enforcement agencies to enforce the rules when there's an incident. And for that matter the only way they are going to be able to identify the drone to match to an operator is if they can actually get there hands on it, which, in most cases will only happen if there is an incident and the drone crashes.
So is it going to prevent someone with malicious intent from getting a drone? Of course not. But is it going to make little Johnny think twice before he sends his drone up into the flight path of a plane? Yes, definitely. But it will be interesting to see what they come up with and how successful they are in rolling it out.
Also, I think once law enforcement is required to respond to "incidents" it's going to open up a can of worms. Your neighbor (someone like nexxypoo) is going to falsely report you for whatever nefarious reason and the police are going to complain they are using up too many resources to respond to unverifiable incidents. Just my two cents...