The intelligence of Neanderthals is the source of a lot of discussion. The level of toolmaking of both species was at roughly the same level and there's some evidence of Neanderthals and humans living side-by-side for quite a while and possibly even forming joint societies. Why exactly Neanderthals went extinct is also not completely certain, some theories suggest humans were better at procreation, other say we were better at the adapting to changing environments. And there also the possibility we simply got lucky.
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 it's often hard to understand why the movements were so important.
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 Williams 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, comedians 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 1990s / early 2000s 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 routine was insanely popular in the late 1970s and early 1980s, and it was widely recognized as fresh and original. Johnny Carson famously gave him the "OK" sign, when Seinfeld 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!"
Seinfeld was also the first person to joke about airline peanuts, now pretty much synonymous with "lazy comedian".
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 to mid-'70s when the computer world was still based on batch processing.
Linux was considered revolutionary because it was a free full UNIX system that could run on a single off-the-shelf PC. Other free UNIX-like O Ses have since 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 smartphones, electronic book readers, tablet computers, etc. probably don't remember the Newton MessagePad, the grandfather, so to speak, of many of these, and direct ancestor of the iPad. Quite a few of them very likely would think of them as unwieldy or clunky, but when they were introduced in 1994, they were far ahead of their 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.
Technologies in general. Remember how a bunch of gigabytes were supposed to take up an area the size of a floor tile? Yeah... take one look at modern terabyte storage devices and hard drives and try not to laugh at those prospectives.
Similarly, science. Theories that were once revolutionary are now things that are taught to anyone with a basic education, and many people who were geniuses of their era and had what was the best and strongest theory at the time now seem to believe things that are ridiculous. Even though we remember Galileo, Newton and Aristotle, it's strange to most people to think how revolutionary their ideas and experiments (thought experiments or otherwise) were and how far ahead of their time they were. There are doubtlessly an even larger number of scientists, mathematicians and philosophers who have been outright forgotten by history because their work is now obsolete or factually wrong, or just became common knowledge and now exists in the work of later people who improved upon their original works.
If you're too used to DVD and Blu-ray, watching a movie on a VHS tape 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. Plus, 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-forwarded or rewound.
And in this day and age, the fact that theatres or rare TV showings were the only way to see old movies seems kinda silly. Recently, people scoffed at movies like The Lion King being re-shown in theatres... whereas decades ago, that was actually not as ludicrous.
Pre-VCR, it was common practice to re-release movies to theaters just before a sequel came out.
The grocery store chain Piggly Wiggly is the 'trope maker' of the modern self serve grocery store, but most people likely have forgotten that. (Even fewer people remember that it was one of the first businesses to use the "just-in-time" model of production, and even one of the first to use the shopping cart, but that's because to most people, a grocery store is simply a place where you can buy food.)
McDonald's consistently ranks at or near the bottom in rankings of fast food franchises despite being the one that first codified the business model of a fast food joint, partially because it can't stand out in the status quo it created.
Still #1 fast-food franchise in profits by a decent margin though, so it must be doing something right.
Similarly, Howard Johnson's and Holiday Inn were two of the first chains to really standardize the concept of a hotel chain as we know it today. Things such as on-site swimming pools, meeting rooms, and on-site restaurants that most motel patrons take for granted became the norm with Ho Jo, but Executive Meddling in the 70s and 80s, combined with a saturation of the market, pushed that chain into near-irrelevance. (Holiday Inn, however, managed to revitalize itself in The Nineties.) Between the saturation of the market and the rampant rebranding present in the motel industry, many facets of the modern hotel that most people don't think twice about were seen as new, convenient, and groundbreaking when they first came out in the 60's or so. A traveler back in the 50's probably would've been blown away by the concept of a motel offering a continental breakfast, for instance.
The Toyota Prius, the first commercially successful hybrid car, almost single handedly launched the Hybrid car revolution... and is now quickly becoming a victim of its own success, as the explosion of hybrid cars it helped launch has given rise to bigger, faster, and even more fuel efficient hybrid cars, with even other cars from Toyota like the Camry Hybrid stealing its thunder, and leaving the Prius with less of an edge, and less to recommend it.
Boeing Model 247, the first modern commercial airliner that first flew in 1933. Unfortunately, it was promptly joined by competitors using basically same technology but were just slightly better—most notably, the Douglass DC-3—that hardly anyone knew about the Boeing plane even in 1930s.
Memes in general. What was considered funny and viral back then can quickly become a Dead Horse Trope when beaten to the ground. For example, the "I was an adventurer like you, then I took an arrow to the knee" meme from Skyrim quickly fell out of grace weeks after the meme was born.
In a similar vein, anything used in a YouTube Poop. Earlier poops, especially those based on The Legend of Zelda CDi Games or Hotel Mario, can be seen as crude or uncreative due to so many poopers (amateur and "famous" alike) using those sources early in the medium's history. Also, most poopers these days have become more advanced at techniques such as sentence mixing, pitch-shifting, what have you, that randomly splicing in a "Gay Luigi" no longer has the same impact that it did in 2008.
Jean Robert-Houdin was a french magician in the 1850's who invented the cliched Victorian magician idea. At the time magicians were seen as beggars, thieves, or children's entertainers, seeing one dressed like the finest of gentlemen, on a stage, performing for high society, was revolutionary at the time, so much so it's still most people's go to image of a magician 150 years later.
David Blaine also fits this trope. At the time, Blaine's minimalist presentations of simple tricks with ordinary objects for unsuspecting people on the street was unheard of. A decade later it's a cliche used by almost every average teen magician as a substitute for actual presentation.
How many people today know that whenever a fictional character delivers some kind of lecture or speech, and at one point exclaims, "You like me! You really like me!", is in reference to an infamous, and almost career-killing acceptance speech Sally Field once gave at the Oscars?