* CrossoverShip:
** In-game, [[WesternAnimation/ThePowerpuffGirls Princess Morbucks]] has an unrequited crush on [[WesternAnimation/Ben10AlienForce Ben Tennyson]], and the prologue manga has Computress being very attracted to Professor Utonium. Not to mention the already existing Blossom/Dexter, and the new [[HoYay Dexter/Ben]], Dexter/Numbah 5, and [[CrackShip Frankie/Eddy]]. Who knows what else there is?
** [[PlayerCharacter Player Character]]/Mandark. Seriously. That's Canon.
** Dexter/Buttercup, which carries on from the old intro tutorial from hints made in older Cartoon Network site games (like Cartoon Cartoon Summer Resort 4 and Toon Hoops) and announcer spiels during late 90s show credits (apparently Dexter likes Buttercup because she's ''sassy''.)
* FandomRivalry: With Bleedman fans, due to fans of ''Webcomic/PowerpuffGirlsDoujinshi'' accusing the artists of stealing his designs after he declined to help with the project.
** FusionFall fans also have a bit of a rivalry with Toontown Online fans, arguably.
* HilariousInHindsight: Some time before ''Fusionfall'' came out, a fan called Bleedman started a doujinshi [[PowerpuffGirlsDoujinshi featuring an Anime-Styled version of the Powerpuff Girls]] and other Cartoon Network heroes fighting an Alien Invasion. Then Cartoon Network starts up a game featuring a bunch of Anime-Styled Cartoon Network heroes fighting an Alien Invasion... To be fair, Cartoon Network hasn't sued him or anything, and both are good in their own right.
** That's putting it lightly; members of the dev team have flat out said that, early in the game's development, they wanted to ''hire'' Bleedman to help do the artwork. Didn't happen, but there's clear similarities in inspiration.
* InternetBackdraft:
** ''[[WesternAnimation/JohnnyTest Johnny]] [[CreatorsPet Test]]''. [[RuleOfCautiousEditingJudgment That is all]].
** Attempts to promote live action shows like ''Unnatural History'' and ''Tower Prep'' or movies like ''Cats and Dogs 2'' through clothing for player characters weren't received well either. The backdraft was ''way'' less than for Johnny Test due to the items being temporary (for both getting and wearing). That and no elements from either series showed up besides the clothes.
* NightmareFuel:
** Dude... [[WesternAnimation/AdventureTime Jake]] isn't moving...
** Some of the Fusion spawn designs are pretty unnerving for an E+ game, for example the {{bear|sAreBadNews}} with a jackhammer for an arm. The concept art on the website is even worse. It's basically a PG version of the ZombieApocalypse with [[EvilTwin Evil Twins]] running around.
** [[WesternAnimation/EdEddNEddy Eddy's]] in-game appearance is either {{Gonk}} or this. Or possibly [[UglyCute neither.]] There's also the disturbing implications of the fact that he's the only [[WesternAnimation/EdEddnEddy Ed, Edd n' Eddy]] character you see in the future.
* TheScrappy: JohnnyTest's presence in the game has not been well received. In addition to being from a fairly unpopular show in general, many people feel his presence is simply out of place because he's not a Cartoon Network original creation like every other character/show presented (though Cartoon Network ''did'' help fund the show's last few seasons).
* TheyWastedAPerfectlyGoodCharacter:
** They could've brought in more stuff relating to TOM, like having to beat off Fusion Spawn who had invaded the ''Absolution'', for instance. And then there are the Hanna-Barbera characters they could've used, for instance, like aiding the WesternAnimation/SwatKats in flying the [=TurboKat=] to take out a Fusion base.
** With the inclusion of [[TheLifeAndTimesOfJuniperLee Juniper Lee]] there were also some interesting locations (Forgotten Falls statue) or other mentions (Te Xuan Ze Inc. in Endsville). These might have been world-building threads from the development team that were never really followed up on, or even explained.
** More like "They Wasted A Perfectly Good Series", in some cases. Some fans feel like the inclusion of certain shows would've been better if certain characters hadn't been excluded. For example, [[WesternAnimation/TheGrimAdventuresOfBillyAndMandy Hoss Delgado, Fred Fredburger, and Jeff]] were included, yet despite their inclusion and references to ''Underfist'' in the game, neither Irwin or [[WesternAnimation/EvilConCarne Skarr]] show up. (Even worse, the former of the two was mentioned, but never seen or heard from, almost playing this straight!)
* UncannyValley:
** [[WesternAnimation/AdventureTime Jake]]. Especially on the homepage.
** The way Abigail's hat leaves a permanent shadow, covering her eyes.