That's exaclty why Microsoft announced a free upgrade offer for all Windows 7/8/8.1 users. Eurogamer has a great round-up on this - they even try to (unsuccessfully) cite Max McMullen's post above:
http://www.eurogamer.net/articles/digitalfoundry-2015-gdc-preview-windows-10-vs-steam-os
You said "there are some obscure
optional features... that Nvidia supports on Windows 8.x but mysteriously not on Windows 7"
Optional features
were introduced in Direct3D 11.1 with
CheckFeatureSupport function - most of those features described by
D3D11_FEATURE_DATA_D3D11_OPTIONS are actually
a requirement on feature level 11_1, so they all will be TRUE when queried on a device opened with that feature level. BUT they were also made available as optional for level 11_0, probably because current NVidia hardware does not fully support feature level 11_1.
HOWEVER since Direct3D 11.1 and feature level 11_1 require a new version of DDI11 that comes with WDDM 1.2, and
WDDM 1.2 is not available in Windows 7 as I said above - there is actually
no technical way to expose these optional features on Windows 7 for NVidia hardware, even though they are fully supported in Windows 8 which does supprot WDDM 1.2.
Well, the same can be said about top feature levels...
Microsoft learned the hard way that levels defined only for certain vendor's cards - that is 10_1 and 11_1 which were only available on AMD cards - are largely a waste of development effort, since no-one codes for them... So yes, it's unfortunate that hardware vendors cannot yet agree on a unified feature set for a new level, but they eventually will.