I've talked several times with a villainous colleague, Dark Aleph, about taking over the world. It's not something I plan to do, but she has some very specific ideas - usually centered around bio-terrorism. I don't think she can succeed, but I humor her. That said, I do think it'd be helpful for new villains to understand why taking over the world isn't something that supers can easily do. Then I'll talk about a group that disagrees with me.
What it means
"Taking over the world" means two things from a practical perspective: eliminating sources of opposition, and guaranteeing compliance with your agenda.
Eliminating sources of opposition requires that you basically destroy the command and control centers of every major world power. And the first step of that process means total nuclear disarmament.
You start with the sovereigns, aka the "nuclear club": the United States, Russia, China, India, Pakistan, North Korea, the United Kingdom, and France. Nukes are also stored in Belgium, Germany, the Netherlands, Turkey, and Italy. Just to be safe, you want to sweep the former Soviet republics. Theoretically these were all transferred back to Russia, but to believe that you'd have to trust the Russians. You also have to scrub Germany, Japan, Sweden, South Africa, Taiwan, Australia, Argentina, Brazil, Mexico, and Canada, all of whom are the proverbial screwdriver's turn away from having nukes.
Why nukes? Because no super has survived the blast of a multi-megaton detonation, and it's very likely that no conceivable super could do so. I know one who survived a kiloton blast, and she was intensely unhappy with the experience. If you really become a global tyrant, someone will find a way to get a suitcase nuke to go off in your face, and that's the end of that. So: no nukes.
Any country on that list is also a country that could mount a significant military opposition to your plans, so at that point you're done. After that, you mostly worry about the intelligence services of Russia and Israel, and the special forces of Germany, Britain, and South Africa.
Oh, and while you're at it, you have to kill every significant superhero out there. Good luck with that.
Guaranteeing compliance with your agenda requires that you subvert the dominant political groups active in a region, or change the culture enough that people will accept you as monarch. The former is far easier in a democracy - you take over the Parliament or the Congress or whatever, install a figurehead from the local population, and go to work. The latter is very difficult; cultural changes happen over generations, not years or months. So my advice is to follow the example of the Romans, who put local leaders in charge and just rolled in once a year to collect taxes.
Overall, you'll find this a very hard sell in democracies where the culture of freedom is strong, like America. Natives of the US will violently resist any outside attempts to control them. You find similar veins of strong nationalism in the UK (look up "euroscepticism" sometime for examples) and elsewhere.
Why it won't work
Essentially, there are too many moving parts in a real government, much less a dozen or more of them. One nuke, one assassin, one crucial betrayal, and it's all over.
Some hardcore bad guys prefer the simplicity of the alternative: depopulation. Kill 90% of the planet, they reason, and you've eliminated 90% of the opposition. Ethical considerations aside, the remaining 10% will include the leaders of government and heads of the military and intelligence apparatus, who have been planning for exactly this sort of attack for years.
The US recently released details of "Project Cycle", a set of operations that were put together between 1962 and 1972 as contingencies against a superhuman uprising. Their planning has certainly gotten better since then, and there's no unclassified reports post-1972 to figure out just what they have up their sleeves.
Even assuming you also killed off every hero that's out there - somehow - it's not like new heroes aren't being born daily. Down the road, you'll be facing someone younger and more determined than you, and possibly tougher.
Who's trying it anyway
Persistent rumors in the villain community talk about an organization known as "the Pact". These guys supposedly are working on a plan for world domination, and they have an unbeatable selling point. "Work with us," they say, "and we'll make you immortal". Don't scoff - according to the rumors, the Pact has some sort of way to resurrect the dead, provided they've been through some sort of initiation process. Even if you commit suicide in a jail cell, they say, the Pact could bring you back.
If true - and frankly, it's so incredible I don't see how it could be - then they represent probably the only group capable of true world domination. But again, it's just a rumor.