Look at the jump there was from iPhone 3GS to iPhone 4. Big step forward.
Honestly I don't see half of that in the step from 4S to 5.
Ok so it's the design changes then...
Spec wise you have to like what you're getting IF the LTE battery life is truly solid.
You would be getting solid battery life on LTE (which is faster than wifi in some cases), slightly better camera, much better CPU/GPU performance, and all inside I still (IMO) sexy design.
Like I saw earlier a week ago in another thread, that iPhone 4 design is something that they definitely want to stick with because of how good it looks. So it's similar to what they do with the Macbook Pros maintaining the similar design each year.
My main thing that pushes me away from the iPhone is iOS and it's lack of innovation. It really brings a stale experience compare to the ROM of jellybean I'm running. They're adding things like turn by turn voice nav and that's great, but there's still a lot of things that need to be done. Second thing would the screen size, which is not "perfect" (just a load of bullshit from people who haven't used a bigger screen size for an EXTENDED period, so they don't know what the hell they are talking about...)