I think it depends a lot of what the idea is. Rewriting a whole fic without permission, bad idea, obviously. But if you read a fic and you see a spell, for one (as it's been stated by others before me already) you don't always know if the spell is canon, something a part of fanon, or that author's specific creation. There's also the problem that you read a fic, and it wasn't anything special for you, you didn't save the link, and you never think of it again, except one little spell or one theory from the fic stuck in your brain. And a year later, you're sitting down to write a fic and it comes back into your head, but without that "I think I saw this in a fic once" note attached, so you have to assume it is your own idea.
Then there's the fact that sometimes two people have the same idea. Sometimes fifteen people have the same idea. I have fic notes that I've jotted down over a year ago and have seen fics pop up since I wrote them down that fic my notes almost perfectly, and they're notes I've never discussed with anyone else. Spells, magical theory, etc. can work the same way. "I need a spell that lets me do 'x'." Well, if someone else writing a fic needs the same spell and you both decide that 'y' makes an appropriate name for the spell and/or translate the same word(s) for the name into Latin, odds are good two people come up with the same idea organically. So if person A posts their fic, which person B never reads, and a year later, person B posts theirs, how does anyone else know whether B took the idea from A or had it themselves? Well, they don't. But I'd lay odds someone will read both A and B's fics and decide B must have stolen it without credit.
If you know where you got something from, yeah, I think it's common courtesy to credit. If you don't, then what can you do? I know there's a bit of a possessive thing with fandom, but honestly? We don't own any of this. We are playing in someone else's sandbox, so do we really have a right to complain if someone else wants to play in ours? If someone comes in and nabs your entire fic and puts their name on it, yeah, I can understand being pissed, because that's flat out stealing your hard work. But when it comes to magical theory, spells, gadgets, and anything else, I think it's sort of fair game. Let's face it, pretty much everyone is inspired for new ideas by what they read, watch, listen to, etc. Just about anything we can think of has probably been done before, and what hasn't, well, it was probably inspired by something else that has.
no subject
Then there's the fact that sometimes two people have the same idea. Sometimes fifteen people have the same idea. I have fic notes that I've jotted down over a year ago and have seen fics pop up since I wrote them down that fic my notes almost perfectly, and they're notes I've never discussed with anyone else. Spells, magical theory, etc. can work the same way. "I need a spell that lets me do 'x'." Well, if someone else writing a fic needs the same spell and you both decide that 'y' makes an appropriate name for the spell and/or translate the same word(s) for the name into Latin, odds are good two people come up with the same idea organically. So if person A posts their fic, which person B never reads, and a year later, person B posts theirs, how does anyone else know whether B took the idea from A or had it themselves? Well, they don't. But I'd lay odds someone will read both A and B's fics and decide B must have stolen it without credit.
If you know where you got something from, yeah, I think it's common courtesy to credit. If you don't, then what can you do? I know there's a bit of a possessive thing with fandom, but honestly? We don't own any of this. We are playing in someone else's sandbox, so do we really have a right to complain if someone else wants to play in ours? If someone comes in and nabs your entire fic and puts their name on it, yeah, I can understand being pissed, because that's flat out stealing your hard work. But when it comes to magical theory, spells, gadgets, and anything else, I think it's sort of fair game. Let's face it, pretty much everyone is inspired for new ideas by what they read, watch, listen to, etc. Just about anything we can think of has probably been done before, and what hasn't, well, it was probably inspired by something else that has.