A game that is "hardcore" is one that "rewards investment"; this is a level of depth, and one that has multiple facets. Right now, I can think of four main aspects to it: time, knowledge, skill, and emotion.
~Investment of Time~
This is shallowest form of depth. Of hardcoreness, if that's a word? Players become invested by the sheer amount of time they have spent. They are rewarded for spending lots and lots of time playing the game. Often, their progress (whatever it is) is directly or indirectly a measure of hours played. Kill X goblins. Collect Y rocks. Accumulate Z experience points. If these tasks require "mindless grinding", then you are investing time when you complete them.
Many MMOs exploit this, of course, requiring huge amounts of time investment in order to reach (and explore) the endgame. Some people love this; some people don't. Some people are proud of their progress, and boast to friends or fellow forum-goers of their time well-spent attaining virtual glory. They're flagrantly hardcore. Others view their progress as a thinly-veiled representation of all the time they've wasted on that stupid game. And some people do both.
Path of Exile rewards time investment, as you've probably noticed. Of course it does. In fact, there are multiple ways in which it does. The most basic being is the XP/level grind. Just by looking at a character's level/XP, a savvy player can estimate the amount of time spent on it. But... even that most basic grind isn't so simple. When someone at the top of the ladder in Hardcore dies, they have to start again from scratch. Yet, the very same person who died at level 90 can come racing back to the top of the ladder (or near the top) again on a new character quite quickly. Clearly, they didn't spend the same amount of time grinding as before. There must be something more to it.
Note that RNG-based systems fall into this category, because players must invest enough time to ride out streaks of bad luck in order find good luck. The randomness simply obscures the correlation to time spent and makes it more fun.
~Investment of Knowledge~
Another way to achieve depth and make a game hardcore is reward knowledge. This where players can progress, or progress much faster than normal, by acquiring knowledge about the game and its systems. Discovering shortcuts, synergies, combos, secret levels, etc. Whereas investment of time most often yields more progress and content, investment of knowledge often results in increased speed of progress.
Another form of this is complexity. By offering up complex systems, players can invest knowledge and learn the optimal paths, the most efficient combinations of moves, etc. that may not be obvious at first glance, even if all the information is there.
In Path of Exile, we have several such systems. The most immediate is the passive skill tree, which is notoriously overwhelming at first glance. It often takes people several characters before they learn how to build effectively for endgame. But we have other, more subtle systems in place, as well. The vendor recipes are a prime example. You can generate a great deal of currency by simply knowing about the GBR 3-link recipe that yields a Chromatic Orb, and regularly checking the shops for cheap equipment. Even subtler, is knowing the best places to grind EXP during a ladder rush. By investing in knowledge, by learning about the game, you can make more efficient progress and gain wealth.
As another example, in a fighting game, you can look at the combo list, but you'll need to learn those combos off by heart if you want to become good at it. Or if you can't (or don't know to) look at the combo list, you'll be at a severe disadvantage compared someone who does know all the moves.
Of course, simply knowing what the moves are isn't nearly enough. You need to be able to actually pull them off. You need skill.
~Investment of Skill~
Some games allow - or require! - an investment of skill. Fighting games, for example, require players to learn and master a variety of moves for every character, if they want to beat the game, or beat other players. This is closely tied to investment of knowledge, but is quite distinct because it is about learning not what to do, but how to do it. This often involves acquiring the muscle-memory to perform a quick sequence of actions, but it can also involve puzzle-solving techniques.
Some puzzles are solved through knowledge, but some are solved through skill. Any puzzle that involves a random initial state will necessitate learning not the solution, but the method by which the solution is obtained. You may know how to solve Sudoku puzzles in general, but you may still struggle with a particularly difficult one. This is something of a grey area, I will admit, between knowledge and skill.
Path of Exile rewards investment of skill. Some bosses require quick reflexes, or careful usage of projectiles or curses. Any monster with energy shield requires some skill, to not let it regenerate. Monsters with reflect auras often require a far more careful playstyle, and reward players (by not killing them) who carefully manage their damage output and healing. Using granite and ruby flasks at the right times is another example.
And, of course, there are races. When players are competing with each other, skill is absolutely vital. Knowledge is, too, but skill is very important. Yes, there is randomness of drops, which are also vital, but that just means skill and knowledge are be-all and end-all. It gives those aren't the best of the best a fighting chance.
~Investment of Emotion~
So the last point I want to cover is regarding the investment of emotion. This one isn't exclusive to games; TV, movies, books, theatre, even music, they also take advantage of this. By having compelling characters and plotlines, interesting stories and worlds can draw in the audience - or the player, in the case of a game - and get them emotionally invested. This isn't necessarily hardcore, in and of itself, but it can be. Ohhhhh, it can be.
Consider Trekkies, Bronies, Whovians, all the die-hard fans of Naruto, Spiderman, Batman, Lord of the Rings, Jane Austen, the Beatles, Justin Bieber, Halo, Call of Duty, Diablo, etc. etc. Are they not hardcore? They're totally into their respective fandoms/cultures/cults, some beyond reason, even. They are all emotionally invested, and woe betide anyone who disagrees with them over the internet.
Much more so than the others, emotional investment is its own reward. Some people like to mindless grind, to kill time. Some people love to learn about new things or master new skills. But everyone loves a good story. It's a great money-earner, too. By getting people emotionally invested in the story, it makes them want to finish the movie/book/game, and it makes them want to buy/play the (inevitable?) sequel.
So with all that in mind, how could a game be hardcore because of emotional investment? Is it even possible to NOT reward emotional investment? You might cite Mass Effect 3, but even so, most people thought that game was great up until the ending. But therein lies the answer.
Games that require emotional investment to the story are hardcore. What kind of game is that? Well, the immediate genre that springs to mind are Visual Novels. They're all about the story, the characters, the plot... literally! If you hate the story, you're not going to finish one of these games. Unless you're masochistically trying to prove something, I guess. The same goes for "movie games" such as Heavy Rain and Beyond: Two Souls. It isn't the tangible gameplay that makes you want to progress, is the story. If you love the story, you'll love the game and finish it. If not... you probably won't do either.
Path of Exile is not terrible hardcore about the story. It is very easy to skip virtually all the NPC dialogue, and there are no cutscenes. There is actually quite a lot of backstory and so on scattered around, if you choose to look for it. Environmental lore, optional NPC dialogue, unique and quest item flavourtext. But this is something we are not "hardcore" about.
So, that's four ways I can think of through which games can be hardcore. There may even be others. I think most hardcore games use a mixture of them, though. I know we do.
