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.
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, and the gratuitously M-rated content comes off as immature. Luckily, the characters 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.
Don't know if there's a better section to put this, or if many others can relate, but an iTunes app was recently released of the Fairlight CMI Series II, designed by one of its inventors. The Fairlight, for the uninitiated, was a $100,000 machine invented in 1979 in Australia which is considered both one of the first major digital workstations and the first commercially available digital sampler. Its "Page R" sequencing was also revolutionary. It was 8-bit, took up a lot of space, but there was nothing else like it at the time. Fairlights were used in The Eighties by many top artists with the cash to buy one, most notably producer Trevor Horn (with Art Of Noise, ABC and Yes), Devo, Paul McCartney, Erasure, U2, Queen, Stevie Wonder, Fleetwood Mac, Elvis Costello, Thomas Dolby, Howard Jones, Hall And Oates, The Cars, Def Leppard, Michael Jackson, Pet Shop Boys, Peter Gabriel, Herbie Hancock, Kate Bush... It basically set the stage for modern synthesizer technology that we take for granted now. In today's world, where $1,000 can get you a decent workstation that can wipe the floor with the Fairlight (which has an '80's charm to it in terms of its 8-bit sounds and the feel of Page R sequencing), the app is getting mixed reviews, partly on being seen as "generic" and "more of a toy".
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 Internet reviewers, especially ones that focus on acting angry and constant swearing (i.e. AVGN, Nostalgia Critic, etc), have fallen into this. While the idea of critics isn't new, it became pretty popular to do video reviewing online. Nowawadays, 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.
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.