I really don't see anything exciting to upgrade for. I'd like to hear why people are upgrading from a 3 to a 4 as well, feature wise.
From what I read it just sounds like upgraded guts, which doesn't do much for me personally.
I've played with the GS3, and the Note II, and the interface is still atrocious with that horrible Samsung skin they put on top of AOSP Android. The scrolling lag is minor, but overbearing at the same time.
The HTC One, on the other hand, has me drooling. If it had a Razr-Maxx like battery, LTE, and was on Verizon, I would be all over it.