It's 'disappointing' in the sense that, although it is more powerful and arguably better than the previous generation, people were wanting more substantial improvements.
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 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
Now about what would they expect from the iPhone 5, that's more complicated. Look, they created an amazing smartphone out of nowhere, then they did the iPad and then they even did the cheaper MacBook Air. I guess people could be expecting something really new again. Can't say it was a smart decision to expect it because iOS 5 is pretty much the same thing but still...
So, now we can all expect an iPhone 5 coming next year by this time but what can they do while keeping iOS 5 around?
So, back to my iPhone roadmap, I can't see Apple releasing iOS 6 just after one year so iPhone 5, to be released in 2012, will still feature iOS 5 thus there won't be magical changes (the ones I was talking about on my previous post). Either Apple do something amazing hardware-wise or they hurry iOS 6 to be a major update to the platform in 2012 already.