It was right about 1995 when every game genre got super-easier for a couple reasons.

3-D replacing 2-D: 3-D games are always easier than 2-D games.  You can move in 360 directions instead of 4.  To make up for this, they take out the precision of crazy jumps.  There are jumps in Super Mario Bros. 1 that are harder than any 3-D game's jumps.  Same goes for shooters.  Games like Halo replaced games like Contra and Metal Slug (although the first Doom and Quake games were hard as hell).

Cutscenes changing the focus to story: now games have stories with endings.  So they want you to finish the whole game and get to the end, so you can see the cliffhanger ending, so you want to buy the sequel to finish the story.  Now it's in their best interest to make the game easy enough to beat.  Back in the day games wouldn't even have endings.  Sometimes they'd just say "CONGRATULATIONS" or "THANKS FOR PLAYING!" or "A WINNER IS YOU!"

There was also an increase in save points, and people have stopped using lives and continues.  Back in the day, if you couldn't beat a game in one sitting, you couldn't beat a game.

I threw in bullet hell games because they're all impossible, indie games because they can do whatever they want and make some crazy hardcore stuff without giant publishers breathing down their necks to throw in aliens, nazis, save points, naked chicks, and cliffhanger endings, and Bejeweled 2 because of that guy who played it for 3 years straight and broke the scoreboard.

Why not? I think an engaging story will help make a game hardcore. I think that because having an engaging story makes a game more complex. But, I dont mean that the story by itself will make a game hardcore. Now, can you explain why a 2D platformer is more hardcore than metal gear solid 4?

Tetris and SMB. Everyone knows NES games were way harder than today's games.

2d platformers and mgs4 exist on the same level. They are core games.

I think rubang was exaggerating a bit when he said that. Not all 2d platformers are on the same level as mgs.

See, mgs is very much a core game, because you play for core-like reasons. You are assigned a score at the end and a rank, you play to advance the story, and there is a skill measurement defined by the game itself.

A hardcore game should be mostly free of skill measurement, rather, it should let the gamer decide for themselves what the skill is. A good way this is done in a game like civilization 4, is that the game can measure any way in which you play it. Prefer fighting? That's fine, there's a way to define how skilled you are at conquering. Prefer diplomacy and culture? Good, same thing. Prefer a mix? That's fine too.

However, in a game like mgs4, compared with something like mario the score only increases when you do something the game wants you to do. For example, you only get points in Mario if you kill an enemy, or get a powerup. There is no score for not killing things and just running through the level. (except the time based score, but you can easily kill things and finish the level in the same amount of time. In fact, only by killing things can you possibly beat it in the fastest time). Same in mgs4. Your rank is based on how few times you are spotted, how few guards you kill, and how much the game is completed.

Anyway, I digress.

Pinball is pretty hardcore, don't you think?

To be honest, "hardcore games" is a marketing term created by developers who want you to think they're "cooler" than... whoever they're comparing themselves to at the time.