The new interface is such a bad idea that they can't even come up with a reasonable name for it.
It was Metro for a long time, but they couldn't come to terms with some German company with a trademark on that term so they dropped it (and said it was "just the development code name").
Then it was the "Windows 8 App" interface for all of something like 48 hours. This was a stunningly bad idea since you have the fine situation where you'd have Windows 8 apps that were not Windows 8 Apps (programs that run on Windows 8 but do not use the new Windows 8 App interface). Imagine the Abbot and Costello routines that their help line calls would turn into.
After that it was the Modern UI. This may have been appropriate in terms of art styles (keeping in mind that the "Modern" style of art covers a period that ended a while back - sometime around 1970) as some of that featured big blocks of color. In terms of interface design it is not "modern" (more like "welcome to 1989").
Now it is the "Windows Store app" interface (the "app" is apparently not being capitalized) . This is only marginally better than "Windows 8 Apps", especially if it is ever possible to buy an application from the Windows Store that runs with the regular Desktop instead of the "Windows Store app" interface. Yes, you'd then be able to buy an app from the Windows Store that isn't a "Windows Store app".
You may see the term "TIFKAM" here and there. That stands for "The Interface Formerly Known As Metro".
I expect that a better name for it would be "Microsoft Window". Just like the existing name but singular instead of plural since apps using the new UI run full screen, one thing showing at a time. That's right, your new CPU can now actually do 4 or more things at the same time but the OS's new UI has been designed so it wants to show you only 1 thing at a time. (Fine on the small screen of a Phone, or probably even a small tablet - silly for anything else.)
They could pick almost any word and it would be fine. Watermelon? OK. Snack? Why not. Flounder? Probably a lot better than anything they have tried using - not only does it describe the thing (a flounder is flat, for a fish it is almost not 3 dimensional, and the new user interface does not use any 3 dimensional appearing things to let you know what is a button or other control) but it also describes what their "brilliant design team" (not to mention the marketing people) have been doing instead of coming up with a name for the thing that is not moronic ever since their first choice was ruled out. "The Flounder UI: if a fish can be flat so can your UI."