Doom's fairly powerful, but not a huge powerhouse. He's got moderate magical power, but not enough to stand up to Dr. Strange one-on-one. He's got fairly powerful armor but not enough to stand up to Iron Man's more advanced armors one-on-one. He's the king of Latveria and has access to nuclear arms and an army of Doombots.
What Doom has more than just about anyone else in the Marvel universe except for Reed Richards, however, is intelligence. He can outsmart just about anyone. He's outsmarted Mephisto, the king of deceit, and he's developed a time machine, various machines for interdimensional travel, and a number of other gadgets on his own. He's got such a strong will that he can resist the Purple Man's mental control powers at point-blank range. The Purple Man himself says that "no one" can resist him at that range. But Doom does without a problem.
He's also the master of the sneak attack. He's stolen the powers of both the Silver Surfer and the Beyonder. He's nearly succeeded in instigating wars through subtle manipulation. He's actually managed to take over the world using a machine that harnessed the aforementioned Purple Man's mind-control powers.
His biggest downfall is his ego. He's arrogant to a fault and that usually clouds his better judgment, especially against the Fantastic Four since, ultimately, Reed Richards is the only man he's jealous of. He gave up control of the world after he'd gained it because he was bored with the menial tasks of running the world. He desires conquest, not administration.