Windows Phone users generally love their Windows Phones. It's got great customer satisfaction numbers. Even so, ever since the company made clear that current Windows Phones would not be upgradable to Windows Phone 8 it's been clear that only a fool would buy the old Windows Phone.
In fact, in retrospect, it should have been clear all along that the old generation of Windows Phones would be abandoned when a new version with code base shared with mainstream Windows came out. This, as well as anything else, explains why both the handset companies (other than Nokia, for obvious reasons) and carriers have been putting very little effort into Windows Phone.
All that should change with Windows Phone 8. It's a very different product, one that has the UI that users love, many new and useful features, and the promise of a far greater supply of apps.
Why will app developers feel differently about Windows Phone 8? The biggest reason is that the shared code base and common programming model of version 8 mean that the development work for a Windows Phone 8 app and an app for Windows 8--the broader, desktop/notebook/tablet version--substantially overlap. Of course there are apps, such as ones that do telephony, that don't have clear analogs on the desktop, but even in such cases there might be common Windows code you can use. Remember, writing Windows Phone 8 apps is writing Windows, so there are lots of programmers who know how.
When Windows Phone 8 was announced I pointed out that the big difference was that it really was Windows.
Earlier versions of Windows Phone had a peculiar programming model based on Windows .NET, probably because the .NET runtime is architecture-independent, so it was a quick way to adapt Windows Phone to different hardware environments, much like Java/Dalvik does for Android.
There's a lot to like about .NET, but it's clearly an idea whose time has come and gone; I've heard for years that the Windows guys at Microsoft hate .NET, and given the way things have gone it would seem that they won and .NET lost. At least old Windows Phone apps still work on version 8, and with many there might be no reason to port the app to the new programming model.
It really should be a simple matter of economics: It's almost certain that Windows 8 will sell hundreds of millions of copies, so you've got very good reasons to write code for it. Windows Phone 8 won't sell anything like that, but if the effort to make an app is marginally more than the work you've already done for Windows 8, you'd be leaving money on the table not to write it.
It's a good time to be a consumer. Strong competition is always good for the consumer.