@elmer

Nth. America:  XP: 26%  Win7: 48%
OK, I can agree with that.  But still a full quarter of all computers no longer able to use the current .net framework?  And in fact anti-supported? 

It seems crazy that Microsoft would cause all these difficuties for 1/4 of all their users.  Though really, they are just causing them for me.  Because I can't say to 1/4 of my user base, too bad, you have to upgrade (not if I want to stay gainfully employed that is).

@cheong

You code in .NET v4.5 and test on .NET v4.0. Or you can just use VS2010 and "code and test" all in .NET v4.0. It's a matter of choice.
Yes I could, and will be, staying on Visual Studio 2010 and .Net 4.0.  But why do I have to be left behind?  Why is microsoft ignoring such a large part of the user (and thus developer) base?
If your application will do "Administrators" tasks, you should expect UAC on Vista+.
You are correct, there are differences between the versions, but these are not subtle hidden issues/bugs.  They are well documented changes/differences.  There is a huge difference between the two.

@AndyC
We do test on XP machines.  But that is not the point.  Testing for bugs that would normally be found at Debug time extends the testing process.  And increases the likelyhood of bugs getting to production.

@ZippyV
I work in the Medical Industry creating applications for data entry.  (That is why this issue frustrates me so.  If these hidden bugs get to our production version then it can impact a patient's care (very bad day for everyone).)