I can see someone being unethical enough to copy n' paste a section of writing from here and claiming it as their own, but ideas?
That's a lot trickier, and it's why ideas can't be covered by copyright law.
A lot of people have written about powerful vampires and their hold on a small region, but no one put quite the imagery, characters and storyline on it as Stephen King in "'Salem's Lot". In fact it was a direct tribute to Dracula by King's own admission, but despite the obvious similarities, the two stories are each distinct and highly regarded in the blood-sucking subgenre.
Epic fantasy has a cozy mold a lot of readers like, wherein an innocent must quest to save the world with a group of companions across a magical landscape. Happens all the time, but each story is unique and oftentimes seen as completely different from it's forefathers.
I don't really worry about it. For one, I'm never gonna post an entire manuscript here, as I'm new and I just wanna be popular.
Two, if I post a sample chapter or an opening, good luck figuring out the totality of the tale. Trust me, you have no idea what my big picture is.
Finally, if I post a query, my manuscript is complete. You don't have a chance in hell of finishing a quality manuscript based on the idea and getting it published before I do. Even if you do, it won't be my story, and if mine is even semi-acceptable, it will read better than yours.
I wouldn't stress it.