I suppose this is the biggest issue for Windows 7 right now, whether it really is a new OS.
While I do agree that for the most part Seven is "what Vista should have been", I do not think it is just an upgrade.
My personal belief is that it differentiates itself enough to be called a new version of the Windows operating system. Why? Because of changes to its usability, for the most part. The new taskbar is just one change, but it is a major one. While us PC savvy folk like us might choose to revert to the old style one(not me personally, but some of us), the fact of the matter is that at least 90% of consumers will end up using the new taskbar, and in itself it changes the way we use windows substantially. Not in the sensethat it is difficult to use(my friend used my PC and she was totally fine using it), but it is definitely a new step, a step up in my book.
Furthermore, there is the inclusion of libraries, which although I'm not sure how much consumers will make use of it, is quite an improvement to the old file system. From testing with my friends, they really like aero peek and put it to use immediately without me even telling them to once they noticed it.
It's not the largest upgrade in the world, but neither should it be: this OS has some of the best compatibility I've seen in an OS transition. I do think MS is trying to cover up for the disdain people have for vista in some way, but I also think they're doing it the right way.
I personally do see substantial performance improvements, and mind you, I had my vista tweaked very well for performance, and even disabled almost all aero effects save transparency and window decorations. I removed gadgets because of performance as well. It was also annoying that whenever I pressed windows+D to reach my desktop, gadgets would dissapear.
Yet Seven runs better at all times for, especially when I have multiple applications(not to mention boot time ^_^). And I haven't disabled a single effect or standard feature of it, in fact only added features and programs that I would normally remove from vista for the quickest performance.
Theres also desktop slideshow, a feature that many I know will appreciate tremendously. And they got rid of the weird dark blueish greenish hue that Vista had throughout the OS(Thank god, I thought it was a horrible color decoration personally).
I just think its too much of a change for a consumer to get through a service pack. Windows XP SP2 is probably the most substantial service pack I've seen(I was too young back in the 95 days so IDR that, if there were any service packs), and it doesnt come close to the upgrade Windows 7 is. SP2 brought some UI improvements and a huge upgrade in security, but again, it didn't change the way I use my computer.
In fact, I honestly feel the transition form Vista to Windows 7 is greater than that from Windows XP to Windows Vista. I personally didn't have any compatibility issues with the transition to Vista, and when it came to everyday use, there was really nothing different save for the start menu and the new arrangement of documents folders. While there were several added features, Aero 3D and pretty window decorations, tons of changes under the hood, a bunch of tweaks and updates, nothing had fundamentally changed the way I used my computer. Nothing made me more productive in my everyday use. Seven makes me more productive, and overall, get at lot less frustrated at my rig.
And mind you, I like Vista, a lot. I just feel it is, in essence, unfinished.
I do just wish they would somehow make it so you can get new window decorations that make use of aero. That would be awesome. I do think a revamped theme would be nice, but its not prioritary. I mean compare windows 95 to 98 to 98SE to ME to 2000, lol. Prettyness wise, they weren't exactly substantially different.
TL;DR(Too Long; Didn't Read version)
So to summarize, Windows 7 helps escape the societal disdain for Vista, while bringing enough improvements and changes to the Windows OS to call itself a new operating system. The most important reason for it to be called a new OS is that it brings a huge change to the everyday use of our computer, I would actually say the largest one since Windows 95. The new taskbar brings a huge change to useability while not being so difficult to get used to, and Aero has now become more than just prettyness, its actually useful(Snap,peek, preview, shake).
Seven is mostly Vista done right, but it's done so right that it deserves to be called a new OS. You can't simply bring about all the changes in Windows 7 in a service pack, and expect everyone to just be immediately adjusted to them. Not to mention service packs download automatically by default.
Think of it this way. Imagine Windows Seven were just Vista SP3 now. Can you imagine a consumer downloading Vista SP3 overnight, waking up and being like "umm... what happened to my taskbar? What is this??! Its weird! Why are my gadgets all on the desktop now?"
They'd get used to it quick, but thats a pretty darn big change for just a service pack.
Performance wise, for us savvy folk its not much, but I firmly believe Seven>Vista for the average consumer with an untweaked or barely tweaked PC. New Benchmarks also support me. Conversation with a friend for whoom I installed Seven on also support me. (He's not as computer savvy as me, but wanted to try seven after I told him of awesomeness)
While new window decs would be nice, look at windows 95, 98, 98SE, ME, and 2000. They barely changed appearance wise, yet were all rightfully called new OSs
Lol, event he TL;DR version is long.