Although some of his short stories are passable, de Camp's Conan is a thinking man, not an instinctive man of action - L. Sprague de Camp and Lin Carter fundamentally changed Conan's basic character.
Don Herron said it best in
Conan vs. Conantics.
A few quotes from that article:
"One of the major points of difference between Conan and Conantics is that REH's creation reacts to dangerous situations instinctively, whereas the de Camp-Carter imitation reacts logically. "
"In "The Devil in Iron", Howard writes: "'Conan. .. is as crafty as a mountain lion. 'It is more through wild animal instinct than through intelligence,' answered Ghaznavi (Conan the Wanderer, Lancer, 1968. p90). " It's evident that Howard's Conan was by and large a man of action. But from the first tale in their chronological sequence that de Camp and Carter write, they make Conan a thinking man's barbarian."
"One sequence in particular completely violates the character of Conan. The Conantics barbarian is cuffed by the overseer on the slave-galley he is a prisoner on. Like the genuine Conan he explodes into action, but unlike Conan he "… belatedly controlled his rage (Conan, p206)." Then the overseer whips him. "But Conan did not scream or move a muscle. It was as if he felt nothing, so strong was the iron of his will (Conan, p207)." If anyone thinks REH's Conan would sit still under a beating… well, they'd better stick to reading Carter's Thongor of Lemuria. It is incredible to me that de Camp and Carter have no more grasp of Conan's character - or of his type of character - than to pass such a scene off on readers."
Don Herron goes on to support each of those assertions (and others!) with proofs from the stories (both from Howard and de Camp).