Let me first put this in the proper perspective. I read hundreds of published books a year, hundreds of manuscripts, hundreds of partials, and thousands of queries. I also read numerous book reviews and have conversations with authors and colleagues about books each week.
As a result, it's pretty hard to surprise me with a new conceit, though you certainly don't need a completely original idea to get published anyway. After all, isn't everything derivative to some degree or another? However, there does need to be a sense of freshness in the voice, a twist that is completely original, an opinion not previously expressed, a subculture never explored, etc.
This problem seems to arise the most when it comes to memoirs. Since the individual has lived the life they're describing, the story will of course feel original to them. They might also feel that the book can be a support for other people who have suffered the same problem. But more likely than not what you're describing has been written about already. So why do a few get published, while most get rejected? A fresh and original voice.
Mysteries are another good example. You'll likely have a sympathetic if slightly flawed protagonist. There's a love interest. Someone dies. The antagonist is caught or killed, which results in a bittersweet victory for the protagonist. So why is John Sanford a bestseller? Because the voice feels original, there's almost always a surprise, and he's just a darn good writer.
So when I receive a query, more likely than not I'll have been pitched or read a similar book in the past. The query should convey both in the style and substance something new, something that jumps off the page.
Easier said than done, I know.