Why would I want to upgrade to the S4 from my S3? I can't see anything here on this S4 forum that would make me want to.
Am I missing something other than the new factor?
I believe that you are.
Understand, the Galaxy S4 was not released by Samsung to entice people to upgrade from their S3 devices. WIll some folks do it? Sure. However, that is not the goal behind Samsung's S4 release. Besides, all device makers release newer models of their flagships, annually.
The majority of people who purchase an S4 will be doing so because, they will be eligible for an upgrade by their wireless carrier and coming from a device that is 2+ years old. That is who Samsung wants.
Samsung has a wide S3 following, that they are very happy with, and want to keep. In addition, Samsung wants to capitalize on that momentum but not by cannibalizing S3 owners making them, S4 owners. Rather, Samsung is hoping to lure customers away from the competition (Apple,LG, HTC) who offer similar products to (in this case) the S4. That's the targeted audience, and the only way for Samsung to increase its market share.
So, if you currently have an S3 and are happy with it, Samsung wants you to retain that device and continue to have a great experience with it. Which is probably why current S3 owners do not see much reason to upgrade.
That being said, I am sure in another year or two, Samsung, will release a device that will be more enticing to the S3 crowd, who in turn, may find it to be a more appealing and worthy of their upgrade.