Metro is definitely evolving. Its pain points (wastage of space, too heavy emphasis on images and text which sometimes clash, monochromatric "too flat" textures n lack of colours) are all being addressed. Some who called this out will be called trolls but the ones defending "old" metro will be left behind as MS itself changes.
Just have a look at the WP home screens from 2 years ago and even upto a year ago. Flat, dull monochromatic front screens, ugly tiles not approaching the vibrancy of even iOS homescreens, let alone Android. What IS metro, is it complete lack of colour as judged by start screen monochromatic look or extreme vibrancy given by picture background as shown in apps (Photos, Music Hubs etc). There were all these inconsistencies.
Have a look at the advertising and marketing once Nokia came in and with its latest Lumia lines (the 520, 720 etc). Goto their website and notice the STARK difference in how the start screen looks. They're now using vibrant colourful varied live tiles to more accurately demo the "alive" nature of the homescreen as in real life. Nokia is doing this to the point where they are pushing default MS and their own apps tiles away from the home screen in marketing (presumably because of their monochromatic tiles) and have put what can only be called utterly random tiles (in terms of functionality, Vogue? Xyz Saint??) but which are quite good to look at.
I personally have large photos tiles, pinned contacts, people tile and Nokia Music tile on my start screen and it looks the most active and vibrant homecreen compared to any other OS or platform, but out of the box by default, I cannot describe how ugly the neon blue tiles occupying my homescreen looked.
The biggest challenges in converting people to WP for me have been getting them to see how the homescreen can be customised to not look so ugly and to show them that is a first class app platform (i.e a "glorified app launcher" is precisely what many people seek)
Wrong. First of all, where are these 100's of millions of Facebook users sampling the WP app? There aren't anywhere near that number of WP users worldwide. The people who see this and other flagship apps are overwhelmingly those who have purchased a WP device. And presumably they purchased that device precisely because they prefer WP to Android or iOS. Do you really think the problem with the original FB or Twitter apps was about metro design language? It wasn't about poor performance or missing features?
The 100s of millions of users are Facebook's users. Users who will never give WP a shot if they don't find FB on WP to be a native comfortable full fledged experience. Crucially, here I mean native to THEIR expectation of FB from everywhere else. Not to WP. The 100s of millions who do come into contact with WP but do NOT give it a shot because they have read or heard or personally felt their apps are not yet first class experiences on this platform. I suspect you like most other enthusiasts have a OS>app notion. This is not what most people want, they indeed do want glorified app launchers. They literally use any phone or OS as a mere platform to get to the apps they want in order to accomplish the tasks they want. This is where the 'FB should be Metro' argument completely falls flat. FB should be whatever it requires to be the best and most comfortable experience for its users. There is no reason it should artificially conform to Metro design especially because if anything, Metro is simple and Facebook is anything but.
Another very important point to keep in mind is Android and iOS did not just forge a unique way out of zilch. WP did. Android and iOS use traditional home/desktop/icons/menus models that were established even prior by Symbian and Windows Mobile (or in a way by desktop OSes like Windows and OS X). So that particular way isn't just "a" way, its the default way. Its what people expect, know and are comfortable with.
Metro is radically new, different and as I have explained before, not necessarily better in ALL ways. Uniqueness and differentiation is only good when it is clearly better. That's business strategy 101 but people just run with the differentiation part forgetting the better part.
If I make you change your workflows WITHOUT demonstrating to you a clear and perceivable benefit, you will complain. Its only natural, you will only invest in relearning and familiarising when the value in it is apparent.
Metro's core apps and general system UI provides a very vibrant n beautiful interface especially for easy/llight UIs. However the reliance on background imagery to provide the vibrance, the large text as chrome, the generous spacing all fall down when you start getting into heavy apps. Can people imagine Metro first person shooters? The HUD in Metro text? Photoshop in Metro? That cluttered toolbar and palette with Metro amount of spacing? Horses for courses. Where Metro doesn't work, adapt the UI and adapt Metro to find a middle ground.
The new FB app does this adequately, if you look at the fine details, the edges are squared (not rounded like Android or iOS), the whole UI is flat without drop shadows and textures, there is no faux 3D, swiping to text heavy areas reveals consistency in fonts with Metro design. Metro is woven into this new FB app as opposed to the old FB app which was puking Metro right at you the moment you opened it. The previous app's only claim was "look I am metro, I am different, look at me, its all Metro". The new Facebook Beta is Metro in all the right places whilst maintaining consistency with FB's other properties (Which is the more important thing).
Another critical things that might help sort out what should and shouldn't be pure Metro: Anything thats small, utility, light, or infrequently used should follow OS design guideless to provide expected behaviour to user and ease transition. Such as Dialler, flashlight, Calculator etc etc
Anything that is heavy, UI intensive, primary app for many people with which they might have extensive prior experience or basically 'more important than OS apps" should optimise around their own workflows: examples of this would be FB, Whatsapp, Twitter, Gaming, Email, Video Conferencing for work.