SunSpider results show that the browser is twice as fast as the previous generation. The processor is twice as fast with a 7x graphics performance improvement. That's a substantial improvement over an already good product.
When you say people were wanting more substantial improvements, what you really meant was that they wanted a different name (iPhone 5) and case design. If the iPhone 4S internals were put into a different case and was called the iPhone 5, we wouldn't be hearing complaints. These complaints are superficial (at best), and the overwhelming initial demand is showing that many people don't find those issues at all.
I think we are arguing the same point: the iPhone 4S is a great product and does have solid improvements.
I am not disappointed with the 4S, but it is apparently clear that a few vocal people are. Why are they disappointed? I can only guess that it would be because it doesn't have a 'next generation' design.
At first I was disappointed because I was after a larger screen (or just more of an 'edge-to-edge' display) for my iPhone and I would happily upgrade to that, but that is the extend to my disappointment. It is evident that the 4S an improvement upon the second generation and will be welcomed by people coming off their 3GS contracts
When you say people were wanting more substantial improvements, what you really meant was that they wanted a different name (iPhone 5) and case design. If the iPhone 4S internals were put into a different case and was called the iPhone 5, we wouldn't be hearing complaints. These complaints are superficial (at best), and the overwhelming initial demand is showing that many people don't find those issues at all.