I think with defensemen it's a little easier to tell if somebody has a lot offensive acumen just with the eye test than it is with forwards. Maybe it's because you don't expect offensive ability quite as much with dmen as with forwards. When i watch Drysdale, it looks offense just doesn't come naturally for him. He may still put up 30-40 points, but i don't see him being anything close to a 'big scorer'.
He doesn't really make plays that would indicate he's got great vision or hands and he doesn't have the acumen or instincts to force himself into dangerous areas with the puck on a whim like you see a lot of the greats doing. And his shot isn't great either.
When he first came up after that nice run he had in the AHL, he had a similar profile to Dan Boyle i thought - smaller RHD, a good skater, but he just doesn't have anywhere near the vision, shot, or creativity that Boyle had