I thought MS said any WP7 app could run on wp8 or win8 metro. And any app designed for win8 metro could run on RT. Did something change or am I not remembering correctly?
Almost...
WP7 apps will run on WP8, but not the other way around (*).
Modern apps are identical for W8 and WRT. The same app will run on both OS' (**).
Modern apps WILL NOT run on WP8, and the other way around won't work either (***).
(*) apparently a small number of exceptions exist where the WP7 app will not run on WP8
(**) identical means the source code is identical. The binary files you download from the app store will differ depending on whether you've got an ARM or an x86 device.
(***) some differences exist. Porting a modern app to WP8 will require the developer to redesign parts of the UI and strip out some functionality only available on WRT. Testing the two different "flavors" of the same app will probably end up being much more time consuming than adapting the source code.
With the exception of (*), this is how Microsoft has always explained it.
Because I know Apple did extensive ergonomic studies and that's why they adopted the 4:3 form factor. I know from my own iPad usage that it's "just right" and wider/taller would be epic FAIL.
As others have already stated, the Surface isn't intended to be a pure tablet. Just the requirement to snap on a keyboard already necessitates a somewhat wider device. Add to that Windows RT's ability to run multiple apps simultaneously side-by-side (the best multitasking implementation on any tablet), and the wider aspect ratio makes even more sense. This is a great example of form following function... just the way it should be.
Finally, until every human on earth is of identical size, weight and build, there is no such thing as a universally ergonomic device. Apple's "studies" on the perfect tablet size (4:3) are not studies... it's called marketing. If Apple ever decides they need a device that can more closely compete with the Surface, then they will redefine 15:9 as the perfect aspect ratio. It's how Apple works... it's how they've always worked.