@Dan, the reason is simple, Intel hasn't released the quad-core Broadwells yet and that's why they didn't update the 15" Retina (or the iMac or Mac Mini for that matter). From what I've seen the WWDC announcement will probably have more in store (and it's usually when they release the higher end machines anyway).
Pooky, I think your analysis is spot on, but I still think that even for a non professional user, _one_ port for everything is not very comfortable. And therefore the price is 1378, not 1299, bc almost everyone needs an additional adapter, and if only to connect legacy devices. And even if this is your first computer, right now you still need it, as there are not many USB-C devices out there, or maybe even none at all. Of course the additional adapter also is taking a away a bit of the minimalist elegance. Thats similar to the mac pro, where many people were forced to buy additional thunderbolt boxes, therefore adding space (and noise).
For the user I'm thinking of I'm mostly thinking of the sub notebook market. Like my mother: she would use the machine for web browsing, MS Office, and Skype. The Skype would be the most intense program she would run. She uses a flash drive maybe once a month (if that), and would connect a display less often. The port she would really use would be the headphone port. In this respect this machine would be a perfect fit, and I think there are many users like that.
And there is another point I sorely miss: where is the magsafe connector? I don't know how many times I, my GF or our cats have stumbled upon the power cord, and I think with at least one occasion my MBP would have dropped to the floor if that connector was one without magsafe. So again, a very useful feature (for everyone!) was taken away and sacrified on the altar of minimalism and thinness. And as I said, if you have this additional adapter hanging on your MB, some of this minimalistic elegance - which I basically also like very much - is taken away. So I still think they have gone too far here.
I think they believe since it draws as much power as an iPad, a normal USB cable will suit fine and a MagSafe would not be justified. Not that I agree but that seems like the logical conclusion.
Here's the issue with the MacBook IMO: it doesn't really fit an actual category bracket with specs and price. I don't even have an issue with the single USB port, and here's why:
This machine looks like, from the specs, the ultimate budget user notebook. It's small, light, has Retina(!), and completely silent. Plus it'll come with 8GB RAM standard (enough for budget users), and 256GB SSD storage. The people this will appeal to (and I believe the focus) is the girl (or boy) who wants something trendy that's not complicated. And the fact that it doesn't have a power brick will appeal to many.
But it is not for the power user. For most people this would appeal to, I could see the adapters not being an issue. People will get the $79 adapter and be done, they can connect the flash drive (which they probably won't use that much), an HDMI display, and still charge the thing. For anyone with any real computing needs, you'll still need the Air, which will still be a faster machine that the MacBook, or the Pro.
And herein lies the issue: it's got budget specs, everything is in its favor to appeal to the mass-market Internet user, and it's $1299!!! That's the same as the Pro Retina 13"!! The price is the problem I see here, because it is basically spec'ed lower than the Air and it costs more than the Air, so the only thing that will actually draw anyone to it is the Retina display. The Air has an advantage in just about every other way, not to mention that you could get a Pro for that price. If the price were much lower, I think it would be a better product.
Making a budget product with a pro price may very well be a big problem for Apple. I like the concept of the product, but the price will turn many away IMO.