I'm generally a nice guy, sure I don't take trades or requests from just anyone... and I rant occasionally. But it takes more than a little pranking or bullying to piss me off.
These things will make me go from 0-10 really quick.
(ALLCAPS means I'm yelling, okay?)

When people use letters or numbers for words.
IT'S NOT "HOW R U GOING 4 PIZZA?". IT'S "How are you going for pizza?" HOW MUCH MORE FUCKING TIME DOES IT TAKE TO TYPE WHOLE WORDS?!

People who make suggestions or requests in comments.
Example: "WOW! You did a good job! I bet Supergirl would look perfect in that!" OR "Awesome! You should try drawing Meg!"
THAT SHIT WILL NOT FLY WITH ME, SIR!

When people misspell names of characters, not counting typos. I mean
constantly.
YOU CAN DO RESEARCH ON THIS SHIT!!! THAT'S WHAT THE INTERNET IS FOR!

Anybody who just tries to be your friend
ONLY so you'll draw for them or ask for advice.

People who do not research a character before drawing/painting them.
IF A CHARACTER HAS CERTAIN QUIRKS... YOU SHOULD FUCKING FIND THEM OUT.
Ask the original artist about them or go through as many previous images/video of the character as you can to get the information behind the character. (No, this isn't about Bouip. I mean ALL characters.)
Also, if you don't give your character any personality at all... SHAME ON YOU!

Flies.
I can't stand being bothered by a fucking fly in the room relentlessly by something that manages to escape every time I take a whack at it. I can't get shit done with a fly in the room... plain and simple.

KIDS!!!
Hold your taters, I'm only talkin' about anyone between the ages of 5-15(possibly 18). Babies are great... Sure, they're shit machines. But that's what they do, they're cute! I love babies.
On the other hand... kids... THEY ARE SO FUCKING ANNOYING! If they aren't asking "Why? Why? Why?" It's prank season! Or maybe it's cuz they whine over not getting what they want, or later think they're the greatest motherfucker to ever come to town.
Yeah, I'm talkin' about deviants under 18. Some are great... but most of them just copycat shit like there's no tomorrow...
Also... Underaged "artists" drawing crappy ass nudes and posting it as art, FUCK YOU.

Old People.
Well technically, I'm just terrified of them. Not any individual old person... Old people are okay with me in pairs or alone. But if I see three or more, I start to tremble.
You can imagine how fucked outta my mind I was when I had to spend a week in Florida with my dad and sister as we visited my great aunt and great uncle... Old folks villiage... I pretty much was scared outta my gourd.

Those nasty ass planet-sized "FanArt" pieces.
WHO THE FUCK NEEDS TO SEE KAGOME FROM INUYASHA OR LOIS FROM FAMILY GUY SO FUCKING FAT THAT IF IT WERE REAL, THEY'D HAVE FUCKING DIED OR HAD BOTH FEET AMPUTATED FROM MAJOR ONSET DIABETES.
I don't mean some people who draw BBW versions, that's fine... It's awesome when done right. BUT DAMMIT TO FUCKING HELL... WHEN I SEE THAT PLANET-SIZED BULLSHIT, IT MAKES ME WANNA KICK A PUPPY!

Sonic-style original characters.
It's gotten to the point to where not only the colored-over ones drive me crazy.
You can get mad at me all you want for this one... but DAMMIT I HATE THEM!
I love the official characters, but I can't enjoy painted over characters OR characters claiming to be original ESPECIALLY WHEN THEY'RE STILL HEDGEHOGS OR BATS OR ECHIDNAS OR FOXES. "oh look my fox has nine tails! nooo mine has twelve! How original am I?" Boo.

People who claim screencaps with self-made changes are art. IT'S NOT.
END OF FUCKING STORY. I can respect vectors to an extent, but even then it's just not fair to claim them as art. I sure as hell bet you wouldn't go to the artist of the manga/anime/cartoon/game and say "look what I did!!! It's so original and great, isn't it?"
That shit really pisses me off.