To be fair we are comparing a 2 seat with a 2+2 and that has always made the 911 special because owners can actually get some small kids behind the front seats, something you can't do in any McLaren. This really shouldn't be overlook as it's a big part of the 911's appeal to many owners, if not something I ever considered which is why mine only had 2 seats!
So to your point I think construction is a big part of it, being a fully carbon monocoque car it is both lighter (circa 80kgs) and stiffer, both of which are good things to be on a sports car. They also seem to have sorted out the steering and they have a feel to them a 911 used to have, but has much less of these days (I am told, I am still yet to drive the 570S due to time, but friends I know who have come from 911's tell me this). They are also quite a bit quicker on the road it would seem and as an experience they are a more exciting car to be in and to drive on track and road (again I need to see this for myself).
My comment is really based on others views it must be said, but having been in a few now they do have something about them. However I don't think they will match Porsche (or Audi's come to think of it) build quality, certainly nothing in the Porsche range matches the V10 but the other thing is you can actually buy one. You can't buy a GT3, GT3 RS, GT4 for anything sensible and this is what has put me off the brand a little and why no matter how good the GT3 is, you can't get one for less than 140K, RS for over 200K so it's got MASSIVE competition at that level.
Let me be clear, Porsche kills it across the entire range, they make superb cars, always have and probably always will. However they have dropped the ball, Cayman/Boxster 4 cylinders are a non event and the new turbos have lost a little something as has the steering, but Porsche really knows turbos so I am sure they will sort it. I just think McLaren is making cars you can buy, is ironing out their initial quality issues, putting on a sensible warranty and making cars that look good, good amazingly well and importantly, 12C aside, retain their value brilliantly.