Intelligence.
Laughing as he bounced among the ruins of Marineford, Doflamingo glanced at the wounded below him and laughed even more. He perched on the top of a flagpole, sill miraculously standing upright despite the damage to the island, and sat and thought.
Whitebeard was dead. That was good. That old relic wouldn't have survived the new age. Neither would that idiot Firefist. Honestly, what was he thinking, going off after Blackbeard like that? Stupid people will not survive the new age, and Ace's actions were clearly stupid.
Unlike, maybe, his brother. Doflamingo liked the strawhat kid. He too was stupid, the fact that he invaded Impel Down was proof of that. But he doubted he was completely stupid. A stupid rubber man wouldn't be able to defeat someone like Crocodile, or fight on par with Rob Lucci, or invade Impel Down.
Yes. Strawhat Luffy was a particularly dangerous foe to the New World. Not him, by himself. By himself he was just a suicidal madman, like his brother and his father. But as Mihawk said, people seemed to like the strawhat. People gathered around him, even the likes of Jinbei and Crocodile, even Boa Hancock, a womam frostier than Aojiki. This was what made him so dangerous. You cannot hope to fight a man of his strength when half your army suddenly deserts you and starts clearing a path for him. Still, making people doing what they didn't want to was a particular strength of his.
Blackbeard? Smart man. Using the world government just to break into Impel Down and increase his forces? Crazy, but smart. Stealing the devil fruit power of a dead man? Crazy, damn near impossible, but smart. Staying out of a battle that shook the world until he could make his move safely? Very smart. Although mocking Whitebeard before ensuring he couldn't, say, cut you down with his massive blade was kind of stupid.
Would any of them survive into the coming new age? Doflamingo didn't know, but it would be fun to find out.