The mobility to evade her Q? Depending on distance, you can almost always evade it by predicting it, lol. Only at close range it's practically impossible. The shield power in general is off the charts. Imagine an Ardent Janna using her E, ult and locket for example. Redemption on top of it and you have 3 health bars restored.
Not to mention Diana's singular delayed burst means she's a Diver/Fighter type of a champion. That's like complaining about a juggernaut one-shotting you because Darius has 5 stacks on you and you're a squishie. No shit, Sherlock he will!