Ethan from Ctrl+Alt+Del, as well as Gabe and Tycho from Penny Arcade, tend to treat their respective hobbies (Videogames and/or tabletop gaming) as Serious Business, although this is probably just the authors poking fun at the "hardcore gaming" mentality.
In Triangle and Robert, cooking and food is serious business. Cuisine magic powers the comic's most fearsome warriors, several of the characters have some sort of mystical cooking skill, most of them are descended from ancient lineages of battle-cooks, and it is eventually revealed that the entire universe is made out of pudding.
"You have wronged innocents, Charles. I formally challenge you to a game of four-square. The loser will be given over to the lords of this realm to do with as they please!"
Mal of Head Trip warns her siblings in the tone and posture of a drill sergeant (even using the words "troops," "soldiers," and "mission") not to talk or make any sound whatsoever while watching the final season of Battlestar Galactica. She is dumbfounded to find that they don't in fact give a rat's behind about the show.
Emeril LeGoinegasque, a supporting character in Achewood, is the president of a club dedicated to the made-up hobby of Trashspotting, driving around on garbage day and building up extensively detailed personality profiles of people they've never even met based on what they throw out. He lives and breathes trashspotting, his character blog (yes, Achewood characters get their own blogs) was all about his trashspotting exploits, and he even had a trashspotting forum for a while. To him and his club, if to nobody else on Earth, other peoples' garbage is serious business.
Emeril's trashpotting even acts as a Chekhov's Skill in one arc, where Philippe goes missing — he manages to figure out where he's going based off a sole discarded can of baked beans.
In a The Adventures of Dr. McNinja story, tennis was created to make sure there is always a champion to battle an ancient death machine in a game of tennis every year so it does not destroy the world. The U.S. government loses a team of Navy SEALs to the tennis temple's security system everytime they have to replace the current tennis champion. Also, they have to get through robot commando temple guards to even get to the temple.
The entire concept is so ridiculous that the doctor, well-experienced at this sort of craziness, loses control and starts laughing hysterically.
In Sluggy Freelance holidays are Serious Business. Bun-Bun actually tries to take over the world by becoming the patron figure of Christmas, Halloween, Thanksgiving, etc.
Science fiction and fantasy, and the fandom that has built up around them, are verySerious Business in Fans!.
Darth Maul and Marluxia have come to blows over smoothie flavors in Ansem Retort.
Maul: [while using Force Lightning on Marluxia] Say it, bitch! MANGO STRAWBERRY!
Collar 6 features a BDSM society that keeps superpowered enforcers on retainer. Apparently the Judiaticizes are authorized to kill people.
It's justified in that it's not just the enforcers who have powers. Think about this for a moment: people who enjoy hurting others and/or hurting themselves, in possession of superpowers based on whether they are sadists or masochists (or both). And not just enjoying a little, either. People who literally GET OFF from either the pain they themselves feel, or the pain that others feel. Without someone to keep them in line, how is that NOT supervillains just waiting to happen?
In Homestuck, the log header in John's dad's PDA declares ties to be Serious Business.
A magazine named "Serious Jester" is for those who really mean it.
Something Positive features the Teddy Bear Liberation Front. Plenty of people in Real Life find the idea of plushie fetishism weird or disturbing, but as fetishes go, it's pretty harmless. The TBLF think it's so disgusting they kill and torture people who make, sell, or use "yiffable" plushies.
In Nectar of the Gods, bartenders are the general focus of the Webcomic, in the Tournament Arc bartending and mixing cocktails is very Serious Business! Where just drinking a specific type of drink can either fix a person's emotional problems in an instant or drive to induced suicide.
In the commentary of thisEl Goonish Shive sketchbook strip, Dan claims that having his characters' hairstyles in flashbacks maintain continuity and be logical is serious business.
The storyline Squirrel Prophet —which takes place in a trading-card game tournament while far more plot-important things happen at the same time— has a sort of Running Gag where several characters think something along the lines of "focus on the card games!" independently.