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.