Well, I think that's only true for the time being. It's similar to how when the iPhone 4 first came out, it was hard to notice a speed difference with most apps, but then when developers started leveraging the 4's increased horsepower, the differences slowly became more and more obvious.
I think you'll see the same thing here, in a few months you'll start seeing a lot of games that simply can't properly run on the iPhone 4.
Also, I think that the increase in horsepower will totally transform the camera and video, almost even more so than the spec bump in the camera did. The biggest thing that hurts the iPhone 4 and 3GS's cameras in my opinion isn't the image quality, it's the slowness of them. It takes several seconds to open the camera app, then it takes a few seconds between each picture. This will more or less be instantaneous on the 4S. WHich will drastically make taking pictures with it more enjoyable. You're much more likely to actually capture the moment you wanted instead of missing it by 5-6 seconds.
I also think you'll see the increase in processor speed and HPSA+ lead to more online gaming with the iPhone. There's some now, but I expect we'll see that exponentially increase, as AT&T customers will be able to game effectively even if not on wifi.
