Dusk Stalker
" Everybody gets a bullet!" -Axton
It really varies from circumstance to circumstance, how important something is the the overall plot and whatnot.
If something exists as a trope, as in a widely known and used/accepted concept, it's not something that can be claimed for one thing ripping off another. What matters is how broad the concept is, and how intricately important it is to the content as a whole.
Dante's Inferno ripped off God of War pretty hard, but it's an acceptable thing because it's a style of gameplay, a concept. Even when you really get into it, how much of Dante's Inferno is really from exclusively God of War? The QTEs to kill stunned enemies, and...maybe the Right Stick for dodging. Light and Heavy Attacks, dial-a-combo...those are old hat, design philosophies as old now to action games as jumping was to platformers.
Nick Simmons didn't rip-off Bleach, he plagiarized it. He took exact panels of artwork, modified them ever so slightly, and claimedthey were his ownit was an homage :/ Plagiarism is what is being cited when people start suing each other. When it's clear and cut points that something was clearly taken from someone else's material.
However, do you know how many filed plagiarism cases are found to be too flimsy to stand up in court? A fair amount actually. Even the most recent case of the writer of Link trying to sue Assassin's Creed (experiencing an ancestor's life with some hi-tech) ended in AC's favor, because the concepts, while slightly similar in most cases, were ultimately way too different in execution and result. A lot of plagiarism cases end up being some dude trying to draw similarities to their own ideas with something established and well-known that aren't really there.
I honestly couldn't tell you how "ripping off" something is okay in general, because I would need a specific example to determine it, and it would only qualify for that example. Most of the time, yes, someone did take an idea from someone else, but it ends up being way more about what they do with that idea that's more important, and the attitude you have about that idea.
Dmc took an idea from They Live, so what? They used it to pretty great effect, and it's not like Tameem is being a little sh!t about it, he said flat-out before the game released that he was inspired by They Live's use of subliminal messages and propaganda, and that's where he got the idea for Limbo's messages. DMC itself took a lot of ideas/themes from the Divine Comedy.
Forgot it has elements of DMC 3 like story in their, Astro Boy(Mundus boss fight same as bot )
and basically some Animesque qualities.
And basically has a Doctor Who esque like atmosphere character wise(Basically Dante, who isn't human or has any trace of Humanity in him ,who looks to Kat to fill the Human role, much like The Doctor has human companions. God I feel like im gonna be sick for just saying anything Doctor Who)