Hey,
Good question indeed. I've been looking for some stunning and innovative Windows software for quite some time too.
Most of them are either from the Mac platform like OmmWriter or where created from the beginning to be X-platform like Tweetdeck, Evernote or Wunderlist. However, some still exist such as Windroplr, Xobni, Hamster Zip,etc.
the main thing is that on the Windows platform there is no strict graphic guideline like on Mac. So most of the less graphical skilled developers end up maybe coding a great software but with no UIX whatsoever thus creating a "crappy" application.
Also, the business models of software distribution on the windows platform are not prone to create quality software. You ever go Freeware with Toolbar & crapware to monetize your application which piss off your consumers or go with a trial version.
In the trial model, to differentiate their software and force the users to buy it, the developers piles up on useless features, that end up making the application more complicate to use. The perfect example are all the "Pc optimiser" and "Media converter" out there on the Windows platform... They basically all do the same thing with a crappy UI :(
Finally, to mass distribute them, they don't use fancy homepage like on mac ( see Panic.com for a good example ) they go to the download.com and softonic.com which are filled with advertising and fake download buttons. This also lower the overall perceived quality of windows Software.
/Shameless Plug
All this is something we noticed here at Allmyapps. We're a French Startup that build a Windows App Store that help users discover beautiful and useful app and also manage their apps in one click like on their mobile phone. We do this through our website at http://allmyapps.com and our Desktop client.
So Windows and Web app developers, hit us up if you want to distribute your app on our platform, It's free to add your app on our store and shouldn't takes more then 5 minutes to submit it. Http://developers.allmyapps.com
As always feedback is much appreciated :)
Cheers!