"Flash isn't allowed in IE in metro-mode on W8"
What gave you this idea? Flash is most definitely allowed in IE11 "metro" mode, both on Windows 8 x86, and 8 RT. I've checked this in-store on a Surface 2. (Incidentally, when did Google dump Flash support on Android? I thought it was only iPads that had a gimped browser)
The user cannot install third-party plugins on Metro IE, but that doesn't mean that Microsoft hasn't shipped the browser without Flash installed.
(The inclusion of Flash on RT is something Microsoft should make a bit more of, especially as it lets users play all of those Facebook games like Candy Crush on their tablets, without needing a dedicated app for each).
As for why certain features are disabled, the simplest reason is probably that they are implemented using code that requires APIs that are only present in Windows8.1.
IE11 is a pretty good browser, though. So far, I've found that the only stuff that "breaks" is using only the "-webkit" or "-chrome" prefixed styles without the W3C standard attributes being specified too, as they're supposed to be ( and yes, I include a couple of my own HTML uis in that category :( )