It's true that Windows 7's changes were mostly internal. But it's not true that they were minor changes from Vista.

Vista had serious technical problems with memory consumption, which Windows 7 addressed. It also had perceived-but-not-real problems due to superfetch and PCs being sold with too little memory or too weak graphics - changing the name may have helped with that.

But the technical changes were real. Windows 7 rearchitected GDI - offloading it to the GPU (AKA WDDM 1.1 / DXGI) - so that using it didn't cause the bits of each window to be retained in system memory. So Windows 7 can open as many Notepad windows full screen as you like, whereas Vista with DWM enabled cannot.

And Windows 7 rearchitected how services load. Vista had introduced lots of features, implemented as services, but it loaded them all on startup and kept them in memory. Windows 7 introduced a mechanism to load them when the features were actually used, saving memory and startup time.

And then there's the list of visible features, which, as you can see, is pretty large: https://en.wikipedia.org/wiki/Features_new_to_Windows_7