Well, anything, really. The complexity and realism of physics and interactions with the world. The complexity of AI routines. Tracking and calculating more things about the world - worlds can be more dynamic and have more interactions. More things and more complex things can exist within the world. It’s hard to give specific examples without talking about specific types of games.
The idea that games will cost 10x as much this generation is absurd. Why would that even be? New developments in API and game development actually lower the cost of developing games. Newer graphics API simplify the process for both programmers and artists. We’re actually going to see a decrease in the cost of certain aspects of making games simply because of the upgrade in APIs.
Now - if having better technology were the only reason that games cost more to make, then why have we seen an increase in development budgets between 2005 and the present? If we’re using the same hardware, and development costs go up, then it’s not the new capabilities of newer hardware that’s driving up the price.
One of the factors is actually the amount of money spent having to try to cripple games to work on old technology. A significant portion of development time is spent not creating new content, but getting that content to work within the rather restrictive limitations of your hardware. Artists have to cull their models to lower the polygon count because the engine can’t handle it. World designers have to constantly prune out little bits here and there from their world because the hardware can’t handle it. All the effort spent to try to get a modern-looking game to run on old, crippled hardware costs time and money. We have to pay more to make sure our games are sufficiently crippled to work.
Yeah, I don’t hate it as much if the dev actually puts some effort into maximizing the PC versions of the game. Back in the early 2000s, we’d have games develop for PC - made into a proper pc game - and then crippled down from there to work on console. Often there’d be a seperate but related game that would be released on console. So games would still use the technology available first, and only dumb them down later.
A few years into the current generation, that flipped. Most games simply became targeted towards the weakest system, so PC games became dumbed down as a consequence. This isn’t just in terms of graphics, but often in terms of map design, world detail, difficulty (when you design certain types of games to be playable with awkward little thumbsticks, they become ridiculously easy on PC, or if multiplayer, then time to kill values become super low), and all sorts of other ways.
Occasionally you see still a developer go through the effort of making a proper PC game, or at least taking advantage of some of the PC strengths - battlefield 3, for example, is massively better on PC than it is on consoles - but it’s the exception now and not the norm.