Good points.. I see much the same start menu vs screen issues. On my main PC, I have hundreds of applications, sorted by type in the menu. Without recalling the name, I can usually find what I'm looking for in seconds. The start screen doesn't allow nesting or grouping, which could deliver something similar. If you know the name, type it -- fine, back to MS-DOS days. But if you don't, you're playing "Where's Waldo", at least once you have a professional working set of apps. It's probably fine for the beginner or casual user, but I spent a few months as a beginner, decades as something more than that. No one needs a UI optimized for the beginner.
They didn't just not learn from the existing Windows desktop, they haven't learned from other very successful tablet UIs. Apple's Palm-like grid of icons doesn't deliver informational display like MS's tiles, but it was fairly efficient for finding apps, given the small tiles. Android improved upon this by making the program launcher a pop-up, and leaving a home screen for docking of often used apps, drawers full of often used apps for fast access, and informational displays with a great deal more flexibility than those of Windows. Apple added drawers of icons as well. These are things you need when you're using a tablet regularly -- the Asus Transformer I'm typing this on has over 200 apps installed.
How would I fix this. First, allow me to sort the tiles any way I like, and use at drawers of tiles, preferably nested. Allow zoom out, at least providing Android-like information density when fully zoomed. On the desktop, I'd make the Start Screen basically a screen saver -- it can automatically show up when I'm idle for awhile in the desktop, showing me useful informational tiles, etc. When I move the mouse or keyboard, the desktop pops right back. Launch from the Start Screen and the restored Start Menu would do exactly the same things. When that screen saver Start Menu is up, some hot key, screen gesture, or other thing would let me lock it in place, rather than banishing it.
That doesn't address the problem that a very, very few applications ever want to be fullscreen on a desktop PC. It's very inefficient for anything involving productivity, most of the time. The old WIndows method of optional full screen/windowed operation works properly. Most types of content creation involve interactions between dozens of applications, data sources, assets, etc. which need to be the same visual context, at least part of the time. Not needed when I'm watching a video, but creating a video? Sure... lots of things going on: graphic arts, audio mixing/recording, video editing, special effects in other applications, DVD or BD authoring, etc. This is broken if every app is fullscreen. Or even just consider the mentioned case of a calculator pop-up. I use a pop-up, not-fullscreen calculator app in Android, because this sort of thing is precisely needed as a add-on to other things. It never needs it own full screen.... why would I devote 1920x1200 (or whatever YOUR tablet rez is) to a simple calculator? Even back in the early days of single tasking UIs, Apple realized some applications were inherently needed alongside others, even in that year or so before real multitaking arrived in GUIs (AmigaOS).