Seinfeld Is Unfunny/Other Media

Fine Art

 * Artistic movements. Numerous ones like impressionism, surrealism, dada and postmodernism. While they were very modern (or even postmodern) and controversial when they were introduced, the fact that they've been around for ages means that its often hard to understand why the movements were so important.

Recorded and Stand Up Comedy

 * The infamous "chicken joke" would be a brilliant subversion of the concept of a punch line, if only it weren't one of the first jokes most people heard. Some people miss the point so completely that they try to make it a real joke -- claiming, for instance, that "the other side" is double-entendre for death. (Link NSFW.)
 * High-end computer technology in general. What's cutting edge can become mainstream and even low end very quickly by other companies doing it a lot cheaper. Just ask SGI or Cray.
 * Robin Williams and his mastery of Rapid-Fire Comedy were once new and exciting. (Mork and Mindy never would have survived without it.) Thirty years later, this same style of comedy has become a punchline in and of itself, most notably in a SNL Celebrity Jeopardy sketch where "he" is told "For the love of God, SHUT YOUR MOUTH!".
 * It was the combination of rapid fire physical and verbal humor that made him famous, like having Groucho and Harpo in the same person.
 * Stand-up comics. Most original and groundbreaking ones seem less so a generation later -- or less -- when their styles and gimmicks are widely reproduced. It can be difficult to understand what makes, e.g., Lenny Bruce or Richard Pryor so important when stages are saturated with comedians who do approximately the same thing, many of them at least as well.
 * In Britain, comics like Billy Connolly and Jasper Carrott were radically different from the traditional working men's club comedians in the 1970s who stood at a mike with a beer in hand and told jokes. Now their observational comedy and conversational style is the norm. Not that they're unfunny now, but they can seem like very conventional establishment figures when once they were radical.
 * Similarly, the work of many of the comedians of the "alternative comedy" set of the 1980s now often looks as quaint as the earlier comics they were reacting against, when viewed in context of later work.
 * Surrealism, non-sequiturs, and a rambling rhetorical style are so widespread among stand-up comics of the late '90s / early '00s that it's easy to overlook how influential Eddie Izzard was when he first coined that style. And even he simply imitated what certain American stand up comedians and Monty Python did decades earlier.
 * Jerry Seinfeld again. His observational stand-up was insanely popular in the late seventies and early eighties and it was widely recognized as fresh and original. Johnny Carson famously gave him the "OK" sign, when he first appeared on the Tonight Show. Fast forward a few years and Seinfeld had become the go-to-impression of a lazy hack comedian, due to being copied to death by lesser comedians. As early as 1985, a recurring sketch on Saturday Night Live featured a group of these, dressed alike and beginning and ending nearly every sentence with "What is the deal with..." and "I wanna know!"

Computing and Internet

 * UNIX seems to fall victim to this trope. Multitasking and on-line documentation are now standard, and the command-line interface is admittedly difficult. But clunky as it was, a person using a computer interactively was a major breakthrough in the early-mid '70s when the computer world was still based on batch-processing.
 * Linux was considered revolutionary because it was a full UNIX system that could run on a single off-the-shelf PC. Since then every UNIX-like OS has been ported to the platform.
 * UseNet. In a world where any schmuck with an Internet connection can start a blog or a message board, this has gone from "groundbreaking innovator" to "place where only spammers frequent" very quickly.
 * The Gopher Protocol, which organized content on the internet into a browseable (by a text menu interface) hierarchy of sites, files and folders, complete with a search engine has been all but lost to current generations of web users as the direct predecessor OF the web, however a decent sized gopher space does still exist on the 'net to this day.
 * People these days, with handheld computing devices such as smart phones, kindles, ipads, etc.. probably don't remember the Newton Message pad, the grandfather so to speak of many of these, and direct ancestor of the ipad and quite a few of them more then likely would think of them as unwieldy or clunky, but when they were introduced in 1994 they were far ahead of the time.
 * They were too far ahead of the time, which is why they were unwieldy and clunky (even by 1994 standards). When Palm came along and did the same thing, with much less ambition, they were unbelievably successful.

Technology

 * Technologies in general. Remember how a bunch of gigs were supposed to take up area the size of a floortile? Yeah...take one look at modern terabyte storage devices and hard-drives and try not to laugh at those prospectives.
 * If you're too used to DVD and Blu-Ray, watching a movie on a VHS seems...weird. The quality might seem grainy, the sound is lower quality, etc, and there's all sorts of damage that could have been done. Not to mention, having to fast forward if you wanted to see a certain scene...and knowing they weren't that accurate as most VCR-systems would play about a second of silence and then pick up when you fast-forward or rewound.
 * And in this day in age, the fact that theatres or rare TV-showings were the only way to see old movies seems kinda silly.

Literature

 * Reading stuff written or set in the past could make people wonder just how the heck we even survived today - because in many older settings, you could hear mentions of people dying of diseases that are today almost completely unheard of. Smallpox, Scarlet Fever, Scurvy, Beriberi, Dysentery, Cholera...Yeah.
 * A YouTube comment on the Animated Adaptation of The Velveteen Rabbit asked why on earth they would burn the boy's toys and beddings after he had Scarlet Fever. At the time, scarlet fever being transmitted by personal effects was considered scientific fact. Science Marches On.
 * Most of the reason Dysentery is known nowadays is because of Oregon Trail.
 * Science Marches On is the related trope for this.