We've heard it all. We see where you're going, we know where you have to end up. We know how you think, and we know a clue when we see one. In short, we're pretty darned hard to fool.
I started reading mysteries at about fourteen. I've been from Donald to Dexter, from Paretsky to Poirot, and honestly, I'm not nearly as savvy as a lot of my friends in the mystery community. But familiarity with the genre means that when you, the author, spend a little too much time commenting on the nosy neighbor or the blue cashmere sweater, I, the reader, pick up on that, knowing it's important. It's like the minor role in the TV cop show that's played by a major actor. You just know he's coming back later in the story.
Conventions have to be used very carefully, therefore. Drawing everyone together to reveal the killer is WAY overdone, and it hardly ever makes sense unless we're all on a train or trapped in an abandoned mine. When it's done these days, it is often with humorous effect, a tactic demonstrated quite nicely by L. C. Tyler's TEN LITTLE HERRINGS. When the author's tongue is out of his cheek, however, it is fake, theatrical and flat for me, and I suspect that's true for many seasoned readers of mystery.
So what can a mystery author do? Whatever plot, scenario, or denouement he chooses has been done somewhere, sometime. If it hasn't, it's probably because it won't work. I think what we must strive for is freshness of character, excellence of style, and precision of plot. Maybe I've read ten versions of the-police-think-I-killed-her-so-I-have-to-clear-my-name this year, but if I like your protag and you tell the story with panache, I'll still enjoy, even when you mention that blue sweater for the third time.