Well, it's looking like a great phone but not that should have S3 owners crying into their beer. All but the most impatient can wait for the S5, I think. Back on topic though, the only feature of the S4 I'd really like to delivered to the S3 is the one I suspect Samsung are least likely to deliver: Knox!
I collect my corporate email on my personal S3 and beloved employer has it locked down like two locked down things in a pod. Of course, in some ways this benefits me (when I'm not wearing my tinfoil hat) but still, I like the idea of Knox.