About K': I think what Musolini said earlier is absolutely right on the money about how he builds meter fast, it ties in to my next point. One of the main thing that really sets him apart from the rest of the cast is his juggle starters. A handful of the cast can do long damaging corner combos, but often time they need drive, stock or both. On top of that, K''s main juggle starter Eins Trigger (qcf+P) into his follows up Second Shoot (f+B) and Second Shell (f+D) have little to no recovery (-3) frames and (-2) frames respectively (for a point of reference, light attacks are usually 4 or sometimes 3 frames).
Let's think about this: he has good recovery for corner traps that setup damaging long juggles, he's builds up meter quick due to safe strings, he's a perfect 1st character since the opponent doesn't have much meter to GCCD or GCAB themselves -- which makes him arguably one of the hardest characters to get out of the corner against. Oh, he can also start his juggles with Narrow Spike (qcb K.qcb K) and his j.CD on counters . Factor in a good Neomax and super good air normals is why he stands apart from the rest IMHO.
Let's not forget that Arcadia October edition dedicated a section on how to fight 4 characters: K', Kula, Mature (v1.0), Iori. I think they left Raiden out coz there's nothing you really can do against him lol.