Really?
Screen: S4 has a larger, higher res screen
CPU: S4 has a slightly faster CPU depending on the exact model.
Ram: 2Gig available on both the S3 and S4
Storage: 64gb available on the S4, would have been on the S3 too but there was no demand (according to Samsung)
Camera: 13mp on the rear now (up from 8), front unchanged
Battery: 2600mah now but needed due to the new screen
Other connectivity: Nothing new
Software: Android..nothing special that's different (that can't be upgraded)
So it's a spec bump, with a slightly bigger screen, which is what Apple did with the 5. The 4S can do pretty much everything the 5 does, but it's more durable, smaller and frankly looks better.
At least with this update people can't make the last claim about Samsung since the case design has not really changed

Sure it will sell well, and has been mentioned it will be a great upgrade for all those S2 owners.