1. Pinball
Nothing will ever simulate the thrill of contesting one's own reaction time against the mechanical designs of a pinball machine. But this once-booming industry is at the withering end of decline. Stern is still going strong, making several new machines (and many old re-releases) per year. But that's like a single rose growing in a concrete lot, as they are the only pinball machine company left in the world. Plus, they've had recent layoffs. Seriously, The Who would never make a Rock Opera about some guy playing Big Buck Hunter blindfolded.
2. Hint Hotlines/Hint Books/Strategy Guides
Before the Internet linked all of our game-solving minds into one massive blob, getting stuck in a game was like wrecking a car. Sure, you might get the thing going again, but it's going to take a lot of work (and probably money).
Speaking of money, hint hotlines were once the second-scammiest thing happening to kids at the time (next to school magazine sales drives). In the 1980s, every game was "Nintendo hard." You would allegedly get your alleged parents' permission and call the Nintendo Game Counselors' hotline for help, which is where the ridiculousness began. I remember the Game Counselors never telling me what I needed to know the first time I called, just giving out ambiguous hints and encouragement ("You have to hunt around there some more, yeah").
On the other side of the spectrum were hint books, which would tell you everything you needed to know, and gave you some really fine writing in the process.
The Internet killed both those things, and only strategy guides remain. With online strategy guides become more and more detailed, paper strategy guides seem to be in decline as well. This is somehow related to people not wanting to pay for things that they can get for free.
3. Joysticks
The icon of gaming for over a decade, the joystick has been relegated to thumb sticks and custom-orders. Even these might be on their way out, with motion-based control systems and real flight control simulators replacing the niche uses joysticks still have. It's not out of the spectrum that future gamers will scoff at physical controllers the way we now scoff at punch card computing.
4. Memory Cards
Though hard disk storage has its advantages, memory cards still hold a special place in the hearts of many gamers. Swapping save files with my Nintendo-playing friends used to not mean sending someone a lengthy code after a totally hassled rigmarole. Right now, none of the three big console systems require a memory card for saving. Some handheld game devices do -- but as data storage technology improves, these will likely find themselves going the way of their console brethren. So future generations won't enjoy the fun of pausing the game for an hour while they run to the store for a memory card.