Windows has had a "Use this computer as a Bluetooth Headset" option buried in the Bluetooth preferences for a long time now.
The Address Book in Mac OS 10.0 and 10.1 would also let you send and receive SMSes with compatible phones (no joke):
http://www.oreillynet.com/pub/a/wireless/2002/11/27/sms.html
And Linux has had a few projects (like nohands) which do the same thing.
It sounds like you underestimate how hard it is to have "UI polish" implemented just right so that it feels seamless and natural instead of a hack (because honestly we do love to hack but ultimately prefer our prototypes to be implemented first-party, e.g I've been using JACK to stream audio in sync on various devices for years but AirPlay is so much more simple and natural)
I'm aware that implementing this well is a difficult problem and I have utmost respect for those who figure out how to make it feel seamless.
Depends which Bluetooth stack you have installed on Windows.
The stack that I know works is the Broadcom / WIDCOMM one.
The closest analogue it comes to my mind is an early demo of Ubuntu for Android in which they showed the ability to reply to SMS messages from within the Unity desktop (although I'm certain I saw a phone call demo too, just can't find a link to the video). However, in that case, both the Ubuntu desktop and Android were running on the same device, so it was easier (still a notable feat for the time).
This kind of integration between different devices is far more impressive, though.
I think the right way to go would be syncing the actions as well as data of Android's rich notifications - things like reply to email/archive from chrome notifications, reply to SMS, hangouts.. It'd be Google's universal apps between Android and Chrome - syncing actions. It'd need a lot of developer support but it'd be fantastic if they pulled it off as well as Apple and supported third parties in a serious way.
A very philosophical question indeed.