So I got the RP installed...
So there is some context sensitivity in the Start Screen's app bar. And I see the "additive" thing where you can right click multiple tiles to select them. Too bad the only context sensitive commands you're left with are unpin and clear selection. I also noticed you can't 'additively" select in the "All Apps" screen.
I could only find one other app with a context sensitivity more focused than "the whole damn screen" however; the Photos app. And all it did was tell me how many pictures I had additively selected. Of course I couldn't figure out what if anything I could do with that selection. Slide show seemed to still show all the photos (and it was over on the right. I'm assuming the convention is context sensitive on the left, though that doesn't seem to be consistent from app to app).
I'm not all that sure what was wrong with Ctrl and Shift+Click for selection; another feature that works in so many places, once you know the keys. Now's where I assume you'll tell me that's a loser feature too because legions of clawing toddlers will knock my hand away from the modifier keys at the last second; therefore this decades old convention had to be abandoned. A more likely explanation is that tablets don't have modifier keys, and this UI is designed for tablets, not desktops. Know compromise.
PS: There still seem to be context menus appearing directly under the mouse in some Metro apps too; usually in a text field for copy and paste, but not always. So much for consistency.
PPS: I was disappointed but not surprised to find there are no file management commands whatsoever available in the Metro file selector screens or in the files search screen. Not even so much as a "new folder" button. Good thing we can pin Windows Explorer to the start screen, because it looks like you're gonna need it a lot if you keep your files organized any more than in a big pile. Maybe someone will write a Metro Norton Commander.
PPPS: Here's a video of me using Windows 8: