It will be a sad day when they force that upon me and I hope its possible to keep that abomination of a start menu disabled... My start screen is setup just how I like it, with the screen divvied up into sections with a mix of shortcuts, internet favorites and different sized live tiles... When the computer is idling or I'm,using a different device that screen is up so I can see at a glance if I get email, if a monitored system,is havi g an issue, the weather, headlines and a little tile with my photos pinging round to remind me that life isn't all work.
As fir a 'jarring' transition when you press the start button, its no more or less jarring than the launch pad in osx which is just a sea of tiny icons, or the android launcher, which again is just a jumble of identical sizes squares.