In the entire publishing history of Marvel Comics, there have been many super villains who've stood the test of time, from the Red Skull (who's been the longest-lasting Marvel villain, having been around since just before World War II) to Magneto (who's made the most on-screen appearances in Marvel movies, thanks to his ever-present role in the X-Men franchise), to Thanos (the most powerful outright villain, and one who's been teased in the Marvel Cinematic Universe leading up to the next two Avengers movies), but there's really only one chief villain in the Marvel Universe. He's the one villain that until a hero has gone up against him, they really haven't had a proper challenge yet.
He's battled the Avengers, Spider-Man, Luke Cage, the X-Men, the Silver Surfer, Superman... he even temporarily stole the powers of the Beyonder! But his chief foes have always been the Fantastic Four.
He is, of course, Victor Von Doom, ruler of Latveria, master of science and sorcery, and the greatest ongoing threat to the planet Earth.
When he first appeared in Fantastic Four #5, little was known of him. His initial plans seemed odd -- in this first appearance, he kidnaps Sue Storm in order to force the other FF members to travel back in time and steal the treasure of Blackbeard, which Doom believed would give him the power to conquer the world, but is easily fooled by Reed Richards. He formed an alliance with the Sub-Mariner to trap and destroy the FF, only to turn on Sub-Mariner as well, resulting in Doom's apparent death. He switches bodies with Reed Richards thanks to a skill he learned from an alien race, but ends up being trapped in the micro verse, which he later takes over until the FF overthrows him.
Over and over, Doom's plots would become more and more grandiose, but then there were times that his plans seemed just plain odd, such as when he recruited four ordinary criminals and then heightened their natural talents in one plot to destroy the FF. Time and time again, he's met his apparent death, only to return, more dangerous than ever.
He's tricked many into allying with him, such as the aforementioned Sub-Mariner (whom he would later force into a partnership), Spider-Man, and even for a short time, the Hulk! He's a master of robotics, as shown in his very first appearance, and nobody is really one hundred percent certain when they meet Doom if it's the real Doctor Doom, or a cleverly-constructed android duplicate, programmed to act as though it is Doom except when in the presence of the real Doom.
It would be two years after his first comics appearance before Doom's origin was shown. His parents were gypsies; his father the leader of the tribe, his mother a witch who died when Doom was young. Doom's father, Werner, was known for his medical knowledge, and when the local Baron's wife was dying from cancer, Werner was summoned to cure her, but it was too late. Fearing the Baron's wrath, Werner and Victor fled, Werner giving his life to save his son. Victor vowed vengeance for his father's death, and later discovered his mother's trunk that held her occult instruments.
Victor grew up to be brilliant, but headstrong. He protected his people from the Baron with his combination of science and sorcery, and would often create devices to trick noblemen into parting with sums of money, only to have these devices fail after departing. Doom's reputation grew, and eventually he was contacted by the dean of Empire State University, who offered Doom the chance to study in the United States, an offer Doom accepted, though it meant leaving his love, Valeria, behind.
Note: A more recent development in this origin had Ben Grimm tampering with the machine due to his dislike of Doom, and blaming himself for Doom's eventual rise to power. I'm not sure I can buy into this idea myself.
Anyway, Doom's machine exploded, leaving Doom's face scarred... well, there was one tiny scar, but that one imperfection was too much for the vain Victor to take. Expelled from the University, Doom travelled the world, later being found by a clan of Tibetan monks, who taught Doom their ways, although Doom ended up mastering them, forcing them to forge a suit of armor for him. His impatience during this construction phase would scar his face more prominently, as the mask had not cooled properly before he donned it.
Now calling himself Doctor Doom, he set forth to avenge himself on those who he believed had wronged him. His first target was his home country of Latveria, which he took over, becoming absolute ruler. Cutting it off from the rest of the world, even SHIELD didn't know who the new ruler was until a later encounter with the Avengers revealed the truth.
Doom's primary target for vengeance, however, has always been Reed Richards and the rest of the Fantastic Four. But destroying Richards isn't his only goal... Doom's endgame has always been the conquest of first the earth, and then eventually the stars. This endgame, to Doom, is because he believes with every fibre of his being that only he, and he alone, knows how to properly rule.
He does have his own code of conduct, however unusual that might be to understand at times. Doom typically does not torture his victims for the sake of torture. If he promises that someone will not be harmed by him, he won't harm them. As the ruler of a country who sees himself as the rightful ruler of the world, Doom thinks of himself as nobility, and that affects his conduct in many ways.
His plans can be difficult to understand, and may at times appear to be pointless, but that just means that the point is lost on us. Doom's plans are not always for lesser minds to understand, after all. A machination on his part may be only to evaluate the potential threat to a later plan of his, so that he can be properly prepared to deal with it when the proper time comes. Once he had an elaborate plan that involved Nick Fury and SHIELD, and they were never aware Doom was behind it all.
At the same time, Doom realizes when an operation has reached the point of diminishing returns, and will abandon it. At least, that's the way it appears to us. Doom may have had the Fantastic Four at his mercy until they turned things around, for example, and he may decide to allow them to leave Latveria, as there's no point in continuing; but who knows what Doom is really thinking? He may well have had that as the eventual end of that plot from the beginning, and accomplished the goal he had -- it's just that this particular goal was not the final destruction of the FF, no matter what he might have said before. For all we know, Doom's posturing about a particular plan resulting in the FF being killed may just be that, posturing... for the benefit of his foes, the FF, who now believe that they've once again thwarted his plans, causing them to eventually start underestimating their foe.
Doom will never admit that anyone is his equal on any terms. He may recognize that someone has more power than he possesses, but at the same time he's already plotting to steal that power for himself, which makes him by default the more powerful. The closest he comes to having any kind of respect for anyone is with Reed Richards, whom he acknowledges is brilliant, but not as brilliant as Doom himself is.
It's this arrogance that is Doom's own vulnerability. Since Doom believes himself to be the best, period, he has a tendency to underestimate his foes (although he'd never admit that to anyone, ever). While Doom has been defeated time and time again, he can never come to terms with the idea that his defeats may have been self-generated. He believes his knows his enemies, but they often turn out to surprise him.
It's a dichotomy of sorts... since Doom can never be completely honest with himself or with anyone else, there is no way to truly know if what he says is the truth, or if it's just the truth as he sees it. He may rant and rave when his plan is undone, but later may try to say that it went just as he originally planned it to go. Since we never know for sure if we're dealing with the real Doom or a Doombot, it's impossible to know for sure what to believe.
As with many villains in comics, Doom can be overused at times, although it's not as bad as it's been for some villains (such as the Joker). I believe that he should only be used in the comics when a really good Doom story is created, and shouldn't be wasted by being used on a regular basis. There have only been a few times in Doom's publishing history when he's been used a lot that really worked, and then it was more of an ongoing plot line that was really part of an epic (most recently, I'm thinking of some of Byrne's FF issues). I'm really not up on what Marvel's done with Doom lately, so can't speak to how he's being used these days.
So far as Doom's appearances in other media, for the most part he's been used badly, I believe. He originally appeared in a few episodes of the 1960s Hanna-Barbera Fantastic Four cartoon, and they never really used him that well. He also appeared in the later FF cartoon (with Herbie the Robot), but I haven't watched those in a while. The syndicated FF cartoon (the one paired with the Iron Man cartoon that was really more of an animated West Coast Avengers, or whatever they were calling that group at that time) used him pretty decently in the second season, as I recall. The animated Avengers: United We Stand probably was the best use of Doom in animation; I don't recall the last FF animated show using Doom at all, although I can't imagine they didn't (which tells you how forgettable I found it to be).
Anyway, that's my take on Doom... if you've got anything to add to this, feel free to leave a comment!