These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Adaptation Distillation: The OVA adds a few more characters to the roster, while downplaying Tadashi and his uncle and making Kaori the main character. Given that the original manga only had four main characters and was relatively short and straightforward, it's natural to assume that more characters would be added to give the OVA more meat to the story.
Kaori herself has parts of her personality removed, specifically her obsessive clinging to Tadashi and her overly sensitive nose, while maintaining the same devotion to Tadashi. Her more negative attributes are shifted to two new characters, Erika and Aki.
Tadashi retains his levelheaded personality even if his role is downplayed in Kaori's favor. Their roles are generally switched, as Tadashi is the one who becomes infected by the gas while Kaori is trying to find him. The two don't even spend any screen time in the same room up until Kaori finally finds him at his uncle's house.
Right before he is attacked by a shark, Tadashi warns Kaori over the phone not to come to Tokyo, as the walking fish plague has spread there. Not that staying in Okinawa would have been any better...
Black Comedy: The scene where an infected Kaori tries to hang herself on the ceiling lamp and gets pushed around in circles by gas escaping through her anus is both a little Squick and slightly amusing
Crazy Awesome: Both the shark-tank and the Professor in general. Not only does he keep his own amputated and RE-ANIMATED ARM as a pet, but also designs a massive, corpse-powered, transforming blimp mecha to house his infected body. Badass.
Also the Circus, who's ringmaster has invented gas-powered acrobatics, gas-powered flamethrowers and a corpse-cannon, powered by the stored gas of dozens of infected.
This manga in general starts out pretty funny, becomes scary, but halfway through the scary part just kind of starts becoming batshit insane.
Narm: It's about zombie fish scurryin' around on little legs. Come on, admit it, it's more than a little silly. Infecting humans may be a step up, but then there's the scene in the OVA where Gyo-ified Erika chases after Aki for something like five blocks. It worked with the landshark... yet now it doesn't.
It's also very hard to take Kaori's over the top reactions seriously since she screams in practically every scene.
The monster fish that started it all was floating around in a garbage bag.
Not just this, but the fact that it floated away from Okinawa and somehow found its way right to our protagonists all the way in Tokyo.
The Scrappy: Kaori due to how annoyingly bitchy she is, along with her whininess well before anything bad has even happened.
Perhaps in an attempt to remedy this (as Kaori becomes the protagonist of the OVA), the OVA's Kaori reacts much more reasonably. She's shown to be pretty much the only one of her friends that actually does anything smart, she's the only one of the three of them that doesn't turn out to be a total bitch and she's extremely devoted to Tadashi to the point of going after him even after discovering he's been infected. She even gets Survivor Guilt at the end!
Ugly Cute: Fish are ugly-cute enough in their natural habitat, but skittering around on land, all confused looking? It's a panoply of cute!