Sure, but the iPhone and the iPhone 3G hardware weren't great. The 3GS was a leap forward in CPU and GPU speed. But everything since then has been faster, but it's not dramatically noticeable.
It's not like the iPhone 4 was having issues displaying web pages, or running apps.
It's not like the 5MP camera was taking horrible photos.
The iPhone 4 has slower 3G download speed theoretically, but it's not like you'll ever reach the maximum speed in a real world setting anyway.
The iPhone 4S is undoubtedly a better phone. And if your upgrading it's a great offer, but it's hard to get excited about something that is so similar to the same model that's been out for almost a year and a half that, as a technophile geek, you can't help but be a little bummed out.