"I should have guessed you were Jace's sister," he said. "You both have the same artistic talent." Clary paused, her foot on the lowest stair. She was taken aback. "Jace can draw?" Nah." When Alec smiled, his eyes lit like blue lamps and Clary could see what Magnus had found so captivating about him. "I was just kidding. He can't draw a straight line." --
"Dr. Watson's summary list of Sherlock Holmes's strengths and weaknesses: "1. Knowledge of Literature: Nil. 2. Knowledge of Philosophy: Nil. 3. Knowledge of Astronomy: Nil. 4. Knowledge of Politics: Feeble. 5. Knowledge of Botany: Variable. Well up in belladonna, opium, and poisons generally. Knows nothing of practical gardening. 6. Knowledge of Geology: Practical but limited. Tells at a glance different soils from each other. After walks has shown me splashes upon his trousers, and told me by their colour and consistence in what part of London he had received them. 7. Knowledge of Chemistry: Profound. 8. Knowledge of Anatomy: Accurate but unsystematic. 9. Knowledge of Sensational Literature: Immense. He appears to know every detail of every horror perpetrated in the century. 10. Plays the violin well. 11. Is an expert singlestick player, boxer, and swordsman. 12. Has a good practical knowledge of British law."
The ability to master other languages (which would have hugely enhanced the scope of these answers). Fully conscious, and either fighting or reciting (or fooling around). The way in which it makes former admirers search for neutral words.
His unique skill had always been to absorb the talents and knowledge of others, use what he needed and discard the rest. He never allowed anyone to get to close. He kept the world at arm's length in order to look down on it.