That Guy never brings his own food, and always steals everyone else's.
That Guy always always does loud and bad imitations when he plays. K-Booom! Dakka-dakka-dakka! Vroom-vroom!
That Guy always smells bad and wears clothes that expose his overweight asscrack.
That Guy never shares his Cheetos, and he always gets orange dust all over everything. He doesn't clean his hands off before touching YOUR minis and books, either.
That Guy will always bring so much cheese to the table that the other players instantly become lactose
intolerant and choke
(fun fact, cheese doesn't have much lactose so this probably wouldn't happen(ALSO choking IS NOT a symptom of lactose intolerance)). BECAUSE THAT'S HOW BAD HIS CHEESE IS
That Guy never bothers to learn the rules, but will happily crash play to a halt every single turn.
That Guy will exploit every loophole in the rules to his advantage or require an utterly anal level of rule following whenever it suits him.
That Guy will try to fight against the party and consider himself clever (because he's "winning"), but will ragequit when the party kills him.
That Guy has tendency to get really mad over nothing.
That Guy fires into melee combat without the proper feats and abilities.
That Guy either doesn't paint his models or paints them too much.
When That Guy attends Magic: The Gathering events, such as a prerelease, he will pay for entry, take the rares from the Prerelease Kit, and leave. Shop owners HATE That Guy .
Hey... FUCK THAT GUY!!!
THAT FUCKING GUY.
And the worst part about That Guy? We all see a little of ourselves in him... and shiver in disgust. The one good thing that That Guy does is move us to be a little better than we are... lest we end up like him.
Sometimes, a bunch of That Guys come together to form That Group a group that only That Guys will tolerate being a part of. The worst part of That Group is that the That Guys that are part of That Group will use its existence to justify their actions to other groups.
The Lament of Humanity
The Ultimate example of 'That Guy'-hood has been achieved. The summit will never be reached by any other. The story of the Marty-Stu, Ao-Sue, Chief-Circle, or "That guy as a GM using a 'universal' homebrewed system across several games". And that's just the tip of the iceberg. Ladies and Gentlemen, read the tale of the one true That Guy... and despair/laugh.
To summarize; the BBEG of ALL Marty's campaigns is an alternate universe version of himself who became a psionic/vampire/shapeshifter that dual-wields lightsaber katanas. Due to having a psychic connection with his other-self, the BBEG can use OOC knowledge to warp reality around the players and thwart their plans (DM changes the setting on the fly to make his railroad as tight as an Eldar's asshole).
He does this in order to bring "Peace" and "Order" to the multiverse by making a setting as horrible as possible and thus his self-insert will be heralded as their Messiah (or their next Mohammed... you WISH I was fucking kidding).
A taste of Marty's retardation beyond the obvious: Using a d20 system, Marty measured his own real life INT score at 18, and made it so that none of the characters being used by the players in his games had an INT higher than that. If the players came up with an idea for something complicated/scientific that he didn't know about off the top of his head, he could say "Your character isn't smart enough to know that or think of that". This man, with "18 INT", thinks that carbon dioxide doesn't dissolve in water (while holding a Mountain Dew), that sniper rifles are the same as heavy pistols, (Just bigger but otherwise same force/impact/etc) and the coup de grâce:
"Genes are like muscles dude, the more you use them the more you get! Then those get passed down to your offspring, like how giraffes have long necks!"
Yes, he somehow thinks Lamarckian inheritance is still a valid model of biology, despite it having been disproven in the 19th century. 18 INT our collective asses.