To be honest, as a PC developer I am rather disappointed by the current SDK preview. I have been hoping for some cool new features and APIs. Especially, I was hoping for interactive tiles, which looked totally awesome in the MS demo videos and would have really added a "wow" factor to Windows. But right now it looks like this won't come with Win10. In fact, despite the universal app stuff, there's barely anything new. A few things which should have been there from the start were added now, like full Drag+Drop support between apps, allowing an app to open folders in explorer, some smaller improvements here and there,... But despite that? Okay, shared storage for publisher apps is also cool. But not mindblowing.
For WP development this is still a cool update, since it adds in a lot of stuff which was already there on the PC, and now finally comes to the phone. But for PC development, I do not see any major improvements, at least not in the current SDK preview. Microsoft was working heavily on the unified platform and on the Win10 UI. It seems there was only little time left for actual improvements for developers. It might still be that MS adds some more features a little later. But with the Win10 release due in summer, I do not put my hopes too high for any major features to appear.
Still, I am really looking forward to Windows 10! So don't get me wrong. I am really excited about all this. With the new UI, windowed apps, unified store, Windows will be a much more attractive platform. This will be great for any app developer, no matter if PC or WP. Also, expanding the device range to include Xbox and HoloLens is really cool of course. So the Windows 10 release for me is more about the platform thing, the new cool UI and expanded device coverage, which should all help bring more users into the platform and into the stores. From the API side there are currently only minor improvements, but that's okay. Maybe they will add more in that regard with some Windows 10.1 or whatever. Or maybe they saved some surprises for //build? We'll know in a few weeks.