An agent/subject of the has gotten his hands on a [DATA: THE EXPUNGING] card that makes card games a serious business: Said card can destroy the other player... or its user, if he loses.
There's also SCP-514, a flock of doves that disables all weaponry in its area of effect and nullifies violent tendencies in everyone around it. This results in teams from rival organizations who are trying to take control of the SCP to use....alternate methods of conflict resolution. Like rock-paper-scissors, riddles, or card games. This has led to the Foundation's commanding officers commenting on how off-putting it is to see two grown men in full tactical gear taking a children's card game so seriously.
In Dominic Fear's Kenny Bassender (Full Title: Kenny Bassender's Quest For Greatness With the Underground Association of Puppydog Racers) movie, Kenny Bassender is a normal person who isn't special. Until he starts playing a game called Puppdog Races, where he is the flawless. So great, that the other members of the Association try to kill him. Not the whole society (it still is in normal present day America), but very serious.
On LiveJournal and its spinoffs, roleplay is very serious business, as evidenced by the "Roleplay Secrets" community, a daily post of (usually) nasty things anonymous roleplayers have to say about other roleplayers, allowed to rag on anything from their characterizations to the size of their avatars. Similar is the "RP Anon Meme", which started as a bi-monthly explosion of hateful anonymous discussion and eventually became a constantly-running community of (slightly less hateful, slightly more spammy) anonymous discussion. People have actually made death threats over pretendy funtime games on the internet.
A meme on the Japanese Internet (a translated version of which is well-travelled on anonymous text boards) involves a somewhat lengthy rant about visits to Yoshinoya being really serious business. Just read it. Variations are popular for ranting about extremely petty things. The meme itself was actually referenced on Sayonara, Zetsubou-Sensei, that's how widespread it is.
Along the same vein, Death Battle is a web series which pits two fictional characters against each other in... well, a battle to the death. The show features fairly extensive research on the combatants involved, but many of the outcomes are taken a bit too seriously by various fans of the losing side. While many complaints are indeed founded into research that could have swung the battle but the team missed (subtle or obvious), others are simply founded on how the critic likes Character X more than Character Y, and other complains are just... not really founded on anything. One of the biggest examples of this has to be Goku losing to Superman. Twice. Dragon Ball fans did not appreciate.
This (subtitled) episode of the Québecquois series "Tom et ses chums" ("Tom and his pals") has the titular character playing a game of D&D with old friends. What he doesn't realize beforehand is that they've kept playing the same party every week during the years he hasn't seen them (meaning they have absurdly high-level characters), and for them (barring the GM), this is very serious business. When they demand that his new character start at level 1 (making him useless since the encounters are tailored for a high-level group), and then belittle him for being a peasant, he gives up and decides to be The Loonie.
Princess Celestia's all-consuming love of tea parties is a recurring plot point. She banishes Twilight to Ponyville for the crime of missing her tea party (never mind that Twilight was busy saving the world at the time). She's been to "ten... hundred... thousand... million!" tea parties in her life, and she remembers them all in excruciating detail. She refuses to cut a visit to Ponyville short to deal with a major fire in Manehattan—but she immediately cuts the visit short upon learning that Luna is hosting a tea party without her.
In "Ponynet Fight!" Twilight is dead serious about correcting others' bad science on the internet. Which just makes it so much fun to mess with her.
The organized crime families take their badminton very seriously. The entire Manehattan branch of the Apple family is devoted to fielding a team.
Noob takes the "MMORPG are serious business" route also, but is much more nuanced than The Guild due to them mostly showing up in reports about video game addiction in mainstream media in France. Skewed Priorities happens a lot and the top guilds ask for people joining them to have a lifestyle that can only happen if you're single and unemployed. At the same time, people that arrive in the top in the domain get enough recongnition to appear in advertisements and live off it. Characters are also seen doing other stuff than playing, while a small portion of the cast is explictly doing it for fun and being able to enjoy the company of their guildmates.
Souichiro: Pray that there's a Taco Bell in Hell! [BANG]
Project: Library has been described as "a Michael Bay movie with books", full of fight scenes and action clichés, but set in a library and whose plot revolves around an overdue book.
In 'Let's Drown Out Night Shift' Ben Yahtzee Croshaw and Gabriel Mortom admitted that the things they said over the weeks have been awful but the ones that seems to get the most attention is that Yahtzee mentioned that 30 fps looks the same or better than 60 fps, causing an outrage in the comments section.
In Rosy The Rascal 15's Locos Doritos video, Jerry eventually goes ballistic over his bag of Doritos being stolen, including up to shooting Amy, who was trying to talk him down.
On Barkwire, rating and reviewing the activities of local dogs is deadly serious.
Jayuzumi tends to run into these players a lot when playing video games and using soundboards to prank others. This happens mostly in Call of Duty, and to a lesser extent in Battlefield.
Mike & Tom Eat Snacks: the Snack is very serious business. People's careers are made and destroyed by how Mike and Tom rates snacks (according to them), and often fans can get upset by their ratings.
In RWBY Chibi, Lie Ren, usually The Stoic and The Quiet One, will take certain things a little overboard — he has a Training Montage for playing tag, scoffs as how Team RWBY pillow fights and blows up at his own team for their inability to dance right.