I have both model years sitting on my desk, and when they are sitting there powered off it's almost impossible to tell the difference. Last year's model is more scuffed but that is merely age and use; had they come out of the box on the same day it would be easy to confuse them. The new model has a slightly blacker shell and the word Kindle is printed in a less-bold font, but just about the only way to tell the difference is to flip them over.
The 2012 model says Kindle on the back while the 2013 model says Amazon (the FCC statement is also different).
But while the differences aren't visible at first once I turned them on I immediately knew which was which. You turned on last year's model by pressing the power button; the new KPW requires that you also swipe the screen.
Update: A reader pointed out that the extra swipe was probably due to one device having ads and the other not having ads. Thanks, Anne!
And once they're on the differences are more obvious - except for the screen. The 2013 KPW has a screen that is at times slightly whiter and slightly grayer than the screen on last year's model. I know it sounds weird but that's what I am seeing.
I think whatever new tech E-ink added to the 4th-gen Carta might still need the bugs worked out because it is not a clear and obvious improvement over the previous generation.
Update: I'm told that the issue I was seeing before was the result of a partial screen refresh. I turned it off and now I think that there is very little difference between the screens.
But once I got to using the new KPW I was able to see how it had been improved. The biggest difference is in the speed. The 2013 KPW is noticeably faster to turn the page, and it is also more responsive in the menus. How much faster?
Well, when I was testing it I noted that the new model was done turning the page before the old model even started to turn the page. It's that much faster.
And then I turned on the frontlight and the difference became even more clear. My 2012 KPW has a very splotchy frontlight, while the 2013 model has a much more evenly lit frontlight. It was also visibly brighter.
These details might not be visible in the photo but they can be seen with the naked eye.
All in all, this is a nicely improved ereader. If you were thinking about getting last year's KPW I suggest that you get this one instead - even if you have to wait for it. I also feel that the new model is worth upgrading to (assuming you can resell or rehome last year's KPW).
Update: My competition isn't all that impressed with the KPW, and he can't really see any improvement to the Carta screen. He recommends against upgrading.
P.S. In the long run the 2 models will probably have the same software features thanks to Amazon's policy of updating old Kindles long after they've been replace. That means the cool new features like Goodreads integration and Freetime (for kids) aren't affecting my decision.
P.P.S. It is very nice. But do you know what would make it better? A 6.8" screen.