I'm still having a difficult time understanding why any user who has figured out how to hit the window key and start typing an application name would find browsing across oversized tiles to be a more efficient application launching experience on a desktop.

Window Key + "Su" + Enter -> Sublime Text 2 is open

vs.

Mouse over on the bottom left of the screen + Click + Wait for Metro interface to pan into view + Wheel over a horizontally scrolling screen of tiles scanning for a "Development Tools" group + Find Sublime Text 2 + Click -> Sublime Text 2 is Open

I don't mouse over to the corner of the screen in all but my laziest moments; but sometimes I happen to only have one hand on they keyboard (perhaps I was just closing a tab in Firefox, or upvoting a comment on Hacker News?). It so happens I'm right-handed, and so my mouse is on my right side, leaving my left hand in the perfect position for the windows key.

My common experience is to press the windows key and mouse around to click. While I proudly tout myself as a keyboard-primary user (and this is still the case. I use the keyboard as my sole tool in many more cases than some of my co-workers), I still happen to use the mouse from time to time and this is an example of it.

Also, when you're using an interface that has such accessible buttons, you tend to learn the general area of buttons, so it's not so much "scanning" as it is "oh yeah, that one repository is about here on the screen, let me swing my mouse over there and click it". The large buttons, again, help make that easier;* so, I don't quite have the same experience.* *

For my less-commonly used applications, I do follow the [windows key] enough text to load a program, usually the whole first word in its name because I type around 90-100 wpm and it's easier for me to type words than sub-sequences of letters [enter]; but it's rather surprising how much I've found myself actually adding apps to the start menu because I find clicking easier. Again, this is just from my personal experience, YMMV.

* I'm not a usability engineer, but I've read an article or two that talks about how difficult actions are for a user, and precision vs speed comes up.

edit:

* * As an example of this experience in progress, you probably know where commonly-clicked icons are on your desktop, if you happen to use your desktop to hold frequently edited documents or shortcuts. If someone re-arranged your icons, you'd be slower because they would no longer be in their previous, familiar locations.

From parent of your response:

    >Mouse over on the bottom left of the screen + Click + Wait for Metro interface to pan into view

Please, please, please, please do something about that bottom left corner mouse crap. I use a Windows Server 2012 VPS via Azure and that is the one thing that drives me insane when managing the server. Not getting a response until my mouse disappears from sight seems so wrong. It sours me on the entire OS almost. I don't understand why the server OS was shackled with the Metro interface, it's horrible for that use case.

You don't have to move to click the little start menu preview, you don't even have to wait for the preview to show up, you just bring your mouse to the corner and click. There's no aiming, just "throw mouse to corner and click". as commented by sukuriant

I guess mousing over to the corner is not so easy when you're in a non-fullscreen remote desktop: the mouse doesn't stop at the corner.

The great thing is that with the Start Screen the first scenario still works and anecdotally is faster for me than the Windows 7 Start Menu.

Actually, on Metro that's:

Window Key + "Su" + Click Sublime Text 2 -> Sublime Text 2 is open

EDIT: sukuriant has informed me that actually you can still just press Enter. So it's EXACTLY the same as before!

Enter still works

Ah, sorry, didn't have Windows 8 to hand, that was from memory. So it's the same sequence of commands!

Correct! I think I would have died if I couldn't still do my favorite win7 practices of "start, type, enter" for EVERYTHING. Ctrl + Shift + enter also works for running the program as an admin :)

"Honestly, I've grown to like it."

One thing I've learned while designing UIs is that if it takes a techy X amount of time to figure something out, it takes between X * 10 to X * 100 for a regular person, so you really want to aim for a super super intuitive interface; and they won't try forever.

>> In all, I can see 41 app tiles without scrolling, both large and small, all of them are easily identifiable and relatively easy to reach.

In practice, I find seeing so much quite overwhelming. Is seeing 41 app tiles - many animated - actually a good thing? My ability to scan and make sense of the Start Screen is quite low. I find it grades on my senses as time passes and every visit to the Start Screen becomes less and less pleasant.

And regarding scrolling, on the desktop are we not used to vertical scrolling? I find it odd when my vertical scroll wheel action results in a horizontal scroll. It's harder to scan while scrolling.

Lastly, regarding have to "dexterously following a series of menus" using a mouse - is it really that hard? Did the MS research say users are unable to use a mouse effectively? I assumed the tiles are huge because a finger is much larger than a mouse pointer, and the Start Screen UI is designed as a touch-first experience.

My start menu has 2 live tiles, 3 if you count the store (it has a little number in the corner for how many apps have updates). I haven't tried with lots of live tiles; and I could see where having too much random activity on the screen could be a bad thing (unless you had them centralized or something; but I haven't tested this use case, so I have no idea).

That said, in my experience, I've only been confused/disoriented/had my senses degraded from looking at too much data when all the tiles are unorganized. If you have a single, giant group with 40 or 50 or who knows how many tiles, and they're all random, that would be, to me, a very confusing scenario. Indeed, if all the tiles I have on my start screen were all in the same group, I'd be confused; but, with groups, they're very easy to distinguish and differentiate. Again, it's like having many many icons on your desktop; or, having icons on your desktop grouped into clusters.

Scrolling. My start screen I try not to scroll. I personally feel that if you're scrolling on your start screen, you're taking too much space and you should tone down the number of quick-access apps you have -- you probably don't use all of them with regularity. That's my personal, private opinion. I am not on the team that worked on the Windows 8 user interface, so I have no idea. That said, I don't know why we're doing horizontal scrolling versus vertical scrolling and anything I said here would be pure speculation. In a column-friendly format, like the start screen, it's not that disorienting, though, because information naturally becomes columns versus rows. That could have been a design feature that came from the horizontal scrolling, I certainly don't know.

As far as the "Dexterously follow a series of menus" and the big buttons on the start menu are concerned, I have to put a disclaimer here: I only have as much knowledge on this as has been displayed on the internet from Microsoft, I wasn't a part of that team. BUT, I do have a grandfather; and I can anecdotal-ly remember him trying to maneuver his mouse and occasionally losing it in menus or miss-clicking because his hand would shake. As computer scientists and engineers and computer/modern-tech savvy people, our hands and fingers are more dexterous than others, I imagine, because we use them for these meticulous tasks all day long. For use cases like that, this new, large-button interface would probably be much easier. I have not had a chance to let him test the UI, so I can't say; and as my co-workers often remind me, my needs and wants are, often, not normal to computer users at large, so I could be very off.