Simon, thanks for starting an interesting thread! I'm getting tired of the "why WP sucks/rocks" threads. Personally, I wouldn't object to a *little* bit of depth; and I mean very little- just enough to help distinguish tiles from each-other. I've only seen W8 briefly in the store so I can't really comment on that, but on WP8 there are times when adjacent tiles will blend together because they match the phone's background. A little depth, or some other visual cue might help separate them in those cases. And I would agree that it would be nice to see MS continue to evolve their visual aesthetic and their design language (within limits). But I'm not sure I would talk about this in terms of the "limitations of 'modern' UI". You like the live tiles and general approach, but want to see some tweaks? Okay, but I would have to disagree that the UI is somehow holding MS back or that it will be their undoing. And it seems you're stretching in a few places to make a bigger argument.
You correctly point out that MS needs to get app performance and features on par with iOS/Android. This doesn't have anything to do with design language. This is one of the biggest barriers.
As far as having a product that you'd be "excited to show to friends", I'd argue that Metro/Modern is much more interesting and share-worthy as-is. I can't imagine anyone in 2013 (or even 2009) saying "wow, check out this grid of icons (iOS/Android) or wow, look at my desktop/launch bar (W7/OSX)". Every other start/home screen and desktop looks the same as it has for years and years.
Regarding 40-55 year old's finding it "childish": well, this is not MS's target (I hope). Gen X is a relatively small demo compared to Gen Y and Gen Z- and MS needs to worry about the latter two. Businesses have no need to upgrade, having just moved to W7. Consumers will not upgrade until they need a new machine (unless they're just real enthusiasts)- phones and tablets have eaten into both desktop/laptop usage and discretionary tech spending. The key is to attract younger buyers and new users.
As far as the change in terminology from "Metro" to "Modern", I don't think this reflects any change in thinking by MS; it is due to legal considerations around the fact that German company Metro AG already uses that word. In fairness, I'm not sure that you actually made any argument about the reason for the change. Anyway, I suppose Modern may be less "limiting" than Metro, but this is all semantics. MS has a consistent, coherent design philosophy. They will absolutely make adjustments to that as they go forward. They'll want to have some immediate visual cues that their next big version of the OS is "new" and "better" (or at least distinctly recognizable as the new one). People get tired of looking at the same thing. Apple has made drastic improvements to both hardware and software over the last few years, but you hear lots of complaints that the iPhone 4/4S/5 look so much alike; and look at how tired and stale iOS itself looks these days, though again there have been constant performance improvements.