Everything about the Innovative Online Industries (IOI), specifically those working under NolanSorrento. Those guys will do whatever it takes to win Halliday's challenge, no matter how horrible their actions are.
The loyalty facilities where IOI forces players to go after they have racked up debt with them due to their entrapment schemes. Players are forced into a tiny chamber and then their VR gear is locked onto their body so they can't take it off. They're then forced to do manual labor in the OASIS until they can pay it all back. Due to compounding interest over time, though, that seems like an impossible task. It's implied that most people die in these facilities due to the horrid working conditions. This is how Art3mis' dad died. And naturally, she has a Freak Out when she's kidnapped by IOI and forced into one of the chambers.
They have trained hit squads both in the real world and in the virtual world to track down everyone they consider a threat to their plans and they will actually kill you in the real world if they manage to figure out your true identity, collateral damages be damned. In an online virtual world where even one slip-up (which can be something as simple as mentioning your real name) means your identity is compromised, this isn't a very hard thing to do. And unless you act quickly, the IOI drones will be all over you with explosives and you won't even know it until it's too late.
And not just you. God help anyone who merely happens to be living in the same Stack, or taking shelter in the same Resistance enclave.
They have all the money in the world to buy all the best items in the game, everything that can give them the edge over other players in their pursuit of Halliday's egg, and what they can't buy, they will employ dirty tactics to get it (like how Sorrento employed I-Rok to take down the previous owner of the Orb of Osuvox). When they have things like the Mechagodzilla and the Cataclyst in their possession, you know they are not messing around.
The book No One Would Listen details how people will threaten and assault stock brokers over tens of thousands of dollars. How much would $500 billion be worth? What would people do for that kind of fortune? In real life, IOI wouldn't have been the only company gunning for Wade.
The "Pure O2 Upgrade" Sorrento planned to implement in the OASIS if IOI had won the contest, which would have filled about 80 percent of the players' screens with obnoxious flashing advertisements. At first it seems like a funny jab at how big, soulless corporations monetize as much of a game as possible to bleed players dry, but it becomes a Harsher in Hindsight when you realize Sorrento's real goal is to totally destroy the OASIS. What looks like a painfully obnoxious tactic to squeeze the player dry is really Sorrento hedging his bets. If he couldn't destroy the OASIS outright, he was likely planning on making the whole thing so painful and expensive to play people would eventually quit altogether. And this was only one of many "upgrades" IOI had planned if they'd won...
The entire The Shining part is as creepy as its original take, especially moreso for those youngsters who have never seen the film before or those who are afraid of horror films in general. Key scenes like the twins in the hallway, the blood-filled elevator, the woman in the bathtub and Torrance with the axe are played to full effect, the latter two basically turning into giant game boss versions of themselves.
True to form, when he's tossed to Parzival as an artifact, Chucky tries to kill him before he's thrown at the Sixers.
The Cataclyst Artifact, which can decimate every single player on the planet its user is on, and there's nothing you can do about it at all once it's activated...unless you have an equally rare Extra Life coin artifact.
It's even worse in the book as the OASIS is divided into sectors. The Cataclyst actually nullifies an entire sector of avatars. If that's not bad enough, remember the real world economy is tied to the game. That's a huge financial loss of value for a large amount of the player base.
The fact that it even exists in the first place is frightening in itself. What was Halliday thinking, putting an item like that in the game?
The only thing scarier than the Cataclyst Artifact is that Halliday made a special worm programmed to wipe out the OASIS. It would destroy, not just avatar, but everything and everywhere in the OASIS. And the only way to release it is by pushing a Big Red Button. Wade came this close to accidentally pressing that button.
The OASIS being a Lotus-Eater Machine. Think about it, the world is a dump. The working class protagonists live in slums, slavery in all but name is openly practiced. Only guys like Sorrento seem to have it good, and that's because he is exploiting people's desire for escape like a drug kingpin. People like Alice's lout of a boyfriend blow their entire savings on upgrades and toys. And because they have this gee-whiz escape from reality 24/7 (only logging off for sleeping, food, and bathroom breaks), they have no reason to improve their lot. Governments and the corporations that might as well be governments have such an awesome circus to dull the citizens that they don't even need to provide bread.