Since no one wanted to jump on this, and I've had a few days to think on it, I'll posit an answer.
Diomedes is distinct in that he is the only hero at Troy to have wounded a god:
"Diomedes, good at the war-cry, drave at Ares with his spear of bronze, and Pallas Athene sped it mightily against his nethermost belly, where he was girded with his taslets. There did he thrust and smite him, rending the fair flesh, and forth he drew the spear again. Then brazen Ares bellowed loud as nine thousand warriors"
Source: Iliad 5.856 et seq. (Murray, 1924)
His ability to best the God of War on the field of battle is due to the patronage of Athena, but it seems to me what makes Diomedes distinct, and allows him to survive the war, are
- the qualities that make him Athena's favorite
It is notable that the other favorite of Athena was Odysseus, who was not always strictly honorable, but was unflaggingly cunning—the Goddess of Wisdom favors the strategists. Wisdom may be said to be more important than honor or any other attribute, certainly to the Ancient Greeks, who gave us Socrates.
Diomedes was held to be a wise and effective ruler in addition to being a gifted commander and valiant warrior.
By contrast, we think of Achilles strictly as a fighter, wholly driven by honor. "Sing, Goddess, of the rage of Achilles" in re: the dispute over Briseis, a slight to Achilles' honor that is the jumping off point for the epic. Achilles in partly defined by his choice of fame over life, knowing that if he goes to Troy, he never leaves.
Ajax is likewise brought down by his inflexibility—the loss of Achilles armor to Odysseus is a slight to great to bear, resulting in his suicide. Ajax remains true to himself, but nonetheless was known for his brawn, not his cunning.
Diomedes was more balanced. He could go to extremes but knew when to pull back. This discernment was useful, especially when confronted by Apollo (who was perhaps less loved by Zeus than Athena, but equal in status):
Diomedes, good at the war-cry, leapt upon Aeneas, though well he knew that Apollo himself held forth his arms above him; yet had he no awe even of the great god, but was still eager to slay Aeneas and strip from him his glorious armour. Thrice then he leapt upon him, furiously fain to slay him, and thrice did Apollo beat back his shining shield. But when for the fourth time he rushed upon him like a god, then with a terrible cry spake to him Apollo that worketh afar: “Bethink thee, son of Tydeus, and give place, neither be thou minded to be like of spirit with the gods; seeing in no wise of like sort is the race of immortal gods and that of men who walk upon the earth.” So spake he, and the son of Tydeus gave ground a scant space backward, avoiding the wrath of Apollo that smiteth afar.
Source: ibid. 5.432 et seq.