What made Peyton so great was he could read the defense, and if needed do audible plays. He wasn't the greatest thrower later in his career, but his ability to figure out what the defense was expecting and his ability to adjust was a huge part of his success.
Ke kept the defenses guessing, and that opened up quite a bit of the field if he and his receivers were on the same page.
Great QB's all have one thing in common: they spend just as much time studying film in their off-hours as they do practicing on the field. It's not just about arm strength, tight spirals, throwing into tight windows. But if you have a QB who does have arm strength, can throw tight spirals, who can thread needles... and is studying film - it's what separates them from the best of the rest.