Here's the thing - at low and mid levels, KoF13 execution isn't significantly harder than SF4 execution, it's just hard in different ways. SF4 has a very link-based combo system - stuff like cr. lk, cr. lp, cr.mp xx special or whatever. KoF combos are almost exclusively cancel based. Command normals can be cancelled into from normals and serve as hit confirm extenders. So a typical KoF13 combo would be something like cl. fp xx command normal xx special, leading into a juggle or whatever. The timing for cancelling is a little tighter than in SF4, but not egregiously so.
At high levels, KoF combos can get really insane. The game comes with a nifty mission mode, where they basically set out combos and challenge you to do them. No big deal, every fighting game nowadays does that, right? Thing is, these combos can take more than twenty inputs. Most of them aren't truly practical, but they get you used to the kind of ridiculous stuff that's possible. Remember, this is a game in which pretty much everyone has at least one 100% damage combo (that usually takes 5 super stocks and a full HD bar).
Basically, the execution is as hard as you want it to be.
--
Besides, marijuana is far more harmful than steroids. - BlitzBomb
I headbang to Bruckner.