But I don't test on many versions anymore, I just test on the latest I have on the machine I am on
You should test on the oldest - that is what is most likely to give you problems.
I have a Win XP VM on my Linux laptop just for IE6.
you watch how things shake up when microsoft opens the windows app store and people begin doing desktop app dev en masse the way they people are doing iphone apps.
I doubt it, desktop apps are a mature market, smart phone apps are not. The opportunities are not the same. I doubt a Windows App Store will make any difference - there are already plenty of places you can download from.
Of course it will make installation more convenient: its nice to see Windows finally get a feature that Linux has had for a decade (except for two typical MS "improvements": more central control and you have to pay for it.)