I don't believe that Windows 7 should cost anything to Vista users, to be honest. It's more like a refinement service pack than a new release - the same as Snow Leopard looks from the list of features shown on Apple's website. I wouldn't call it a major release in either case. In the same way that Intrepid Ibex (8.10) was only a minor update to Hardy Heron (8.04) in the Ubuntu world. Jaunty Jackalope (9.04) was a bigger release, in that it offered Ext4 (but not as the default file system), but still not a 'major' one in my eyes.
Major releases would be the difference between Windows NT from Windows 9x, OS X from OS 9 and Debian 5.0 from Debian 4.0 (or a derivative distribution changing its base distribution, i.e. going from Debian to Ubuntu (though that isn't huge, since Ubuntu was derived from Debian in the first place) or on a bigger scale, moving from a Debian-based system to a Red Hat- or Mandriva-based system). :S
This discussion will never end purely because our views on software (and licensing and open- or closed-source) are always going to be different.
A Major Release is defined by the developer at their own discretion. OS X 10.6 is an under-the-hood re-write of OS X's performance subsystems, and adds new features to make OS X run faster and more efficiently - in the forms of Grand Central Dispatch and OpenCL. Furthermore, it expands 64-Bit support, allows true End-to-End 64-Bit running and features re-writes of all the major Apps included with an OS X install (Bar iTunes*, for legacy reasons) in 64-Bit Cocoa forms.
Whether it seems it from a casual observation or not, Snow Leopard is a major release. Leopard set the new Base for OS X's features and UI. Snow Leopard is aimed at refining how the next generation of OS X releases function at a lower level. A lot of thought and development work went into it.
Windows 7 is a user-facing upgrade to Vista, refining the User experience and making some minor changes under the hood to improve performance, and in some cases leaving the code be to improve compatability. It also has changes to improve reliability. The Kernel code has only seen a .1 upgrade, but the user-level upgrade is significant. It too is undeniably a major release. It has the first true improvement to Microsoft's "Taskbar" User Interface Element ever issued (Previously, only tweaks have been added).
*Our hope and understanding is that a Cocoa 64-Bit iTunes for (At the very least) Snow Leopard will be issued as part of the iTunes 9 upgrade expected sometime in September.

The Frome Flyer: Smoother, Smarter, Calmer,
Winner.
Jenson Button: Professor, Chauffeur, World Champion Racing Driver.