The Abridged Series phenomenon. When LittleKuriboh debuted Yu-Gi-Oh! The Abridged Series, the format of re-editing a series into mini-episodes and the style of quirky, self-referential parody were fairly original and amusing, and while Gag Dubs were certainly not anything new, YGOTAS massively popularized them. Nowadays, there are so many imitators that neither the format nor the humor seem innovative anymore.
Adobe Flash movies. Browse any website full of them and then point out how many cartoons that were "hot stuff" back when they were made now seem rather generic, loosely drawn, or poorly animated. Especially sites like Newgrounds.
Many of Newgrounds' flash games suffer from this, as well. In a time where the site is full of much more complex games, looking back at a time of arcade-y shooting games can be rough for newer users. Stamper even mentions this in the description of his 2000 game Street Life, which was a top rated game at the time, telling new people that it's not worth playing by today's standards.
Not to mention Newgrounds' Series Mascot, Pico. His debut was the pinnacle of Flash 3 programming and was what put Newgrounds on the map. Nowadays, the animation is very low quality, the game-play is incredibly basic compared to what modern Actionscript can do, and the gratuitously M-rated content comes off as immature. Luckily, the characters themselves grew with the times.
YouTube Poop even. Granted, some YTPs are still quite as funny today as they were back in mid 2007 when the fad was new (like where Link decides to toast spaghetti for dinner and Hilarity Ensues), but some jokes have been used so much people may think "Oh, look at the use of the word 'Come', and Robotnik is saying something that sounds like Penis" when they view a few YTPs that came up with it.
Some other early YTPs that were simply screwing around with Windows Movie Maker or other such effects also come off as boring today because we've only heard everything in G major by this point.
Diary Of A Camper is a Quake movie, the first Quake demo with an actual plot beyond simple gameplay footage — and the very first Machinima movie ever made, thus a launching point for an entire new form of art. Its success in the Quake community quickly spawned a lot of other movies from other people. Special websites for reviews of Quake movies cropped up soon. ...And Diary Of A Camper nearly universally received very low scores there, due to how primitive it was compared to what came afterward.
Not to mention, Red vs. Blue was one of the most original uses of games at the time, helped to make Machinima popular, too. While still going strong, it doesn't look that new.
GeoCities. Think about it — anyone, even you, can make their own site on this new, exciting "Internet" thing, and write anything they want, for the entire world to see! Geocities is the place where early Internet culture bloomed. Today, it's mainly remembered (and derided) as that deleted webhost with all the cheesy MIDI background tunes and ugly layouts.
The That Guy With The Glasses One-Year Anniversary Brawl. It's now fully expected that several of the site's members will get together to make a movie celebrating an anniversary, but at the time it was a complete secret what was going to happen, and just the sight of so many contributors in one room was absolutely mindblowing. And of course, now it looks downright primitive with the lack of any plot besides everyone fighting, and lasting only 20 minutes rather than the feature length extravaganzas of Kickassia, Suburban Knights, and To Boldly Flee.
Video Review Shows, especially ones that focus on acting angry and constant swearing (i.e. The Angry Video Game Nerd, The Nostalgia Critic, The Irate Gamer, etc), have fallen into this. While the idea of critics isn't new, it became pretty popular to do angry video reviewing online. Nowadays, it's considered a dated trend since everyone has copied them, and many people consider the recent reviews of the people who originated the idea lost in a sea of imitators and mediocrity.
Numa Numa Dance, one of the first videos on Youtube to go Viral. Seen today, it's "just another fat guy dancing video". No. It's THE fat guy dancing video.
2 years ago, Lets Plays were generally evolving. Nowadays, its usually just a guy shouting old memes and swears, and thus the popularity has dwindled for many.
Creepypastas can fall into this too. Some stories were very original and genuinely creepy when they first came out, but when more and more copycats attempt to use the same plot points (Hyper-Realistic Blood!) they lose their edge and make the first stories look predictable.
In the very early days of online video game walkthroughs, a GameFAQs user known as Kao Megura (whose real name was Chris MacDonald) was an absolute legend. He became famous for his Final Fantasy VII FAQ, which was the game's first English walkthrough, and thus, the one most people clicked on when they looked it up- and besides that, it was very organized and well-written. He wrote many more guides in the same style, and gave lots of advice and assistance to other FAQ writers.... but since literally everyone used his ideas, there is no longer anything special about his guides. MacDonald died in 2004, so most gamers haven't even heard of him, even those who are very active in the GameFAQs community.
GameFAQs in general, and other websites dedicated to written walkthroughs and cheatcodes, got hit hard with this with the rise of Youtube. Once it became popular to upload recorded gameplay online, gamers took to watching video walkthroughs instead of reading.
Once upon a time, an ambitious user of Garry's Mod made the first Stylistic Suck movie from the program. Suddenly, everyone thought this was the only way to animate in Garry's Mod. However, after some genuinely good uses of the program, most people started to see "Heavy flails his arms around and dies" videos as old-hat, or at least from a bygone era.