I think Donald bowled a very good in swinger, which at his pace was hard to keep out. For left handers that became the out swinger which became even harder, and he was also very good at letting the ball go across the left handers. I remember he gave plenty of problems to Saeed Anwar by just letting it go across with the angle to the left hander. Probably also why he was so good against Lara.
You are correct. Inswinger was his natural delivery. In addition to Donald's pace, his height was also a big factor. He was
6'4" tall (a good 6 inches over Steyn), which meant he was naturally high in delivery.
That pace, at that trajectory, made it very hard for batsmen to get on top of his short balls (it was very risky to take chances on those deliveries in Test match cricket with no field restrictions).
That height & pace also meant that - on fast bowler friendly pitches - he could get the ball right at the batsman's throat even by pitching it just short of good length.
Those deliveries were also very hard to deal with, especially for aggressive batsmen who instinctively would try to play at it, instead of cleanly evading it and letting the ball go to the keeper.
There is a video on YouTube on Donald's bouncer-vs-hook shot contest with Australian left-hander Matthew Elliot, Donald was almost always the victor at the end of those battles.
In a sense, Donald had very limited variety but he was super effective with what little variety he had. In the end, that's what matters. Needless to say, he was a very aggressive fast bowler.