Unfortunately, not all of these answers are as demystifying as they could be. I've tallied up three of my big peeves after the jump.
Who - The Implausable Culprit: In mystery, we expect the unexpected. The pool of suspects is laid out for us, and we ponder their guilt or innocence along with the heroine, while knowing that there's every chance that she (and we) are totally wrong about the who, even if we have the what and the why. But when you reveal the murderer to have been someone who was never a factor or never had our attention - and shouldn't have from the clues you gave us - you've struck out. "The killer was... MISTER ROGERS!" Wait, what? Who? That old guy who lived across the street who we saw getting the newspaper once and never again? Lame.
What - The Uncompelling Crime: Sometimes, we're not entirely sure what is actually happening, but that's not a bad thing by any means. It lets us (and our heroine) get our investigation on. There's blood spatter at a scene - is this a murder or an assault? Magical practictioners are going missing without a trace - are they dead, gone into hiding, or has something more nefarious befallen them? Traces of black magic linger at odd places where nothing dire seems to have happened - what harm has this spell caused and to who? If all we have are symptoms, the pay off for what's actually going down has to both make sense and be something that is proportionately dire to those symptoms. When all of these frightening or disturbing clues add up to an endless plague of self-replicating rabid marmots being loosed upon the city, you have lost me.
Why - The Complex Motives Infodump: Thrillers have led us to understand that people sometimes have odd motivations for what they do. (Psycho, anyone?) If I've been following along with your heroine and we're at the end of the book, I probably know what happened and I have some idea as to who did it, but I may be a little murky on the why. That's fine. Sometimes, the 'why' doesn't really matter or even lessens the impact of the story (again, Psycho, anyone?). However, once Mister Rogers is unmasked as having been the cause of untold destruction and he was never even on our radar, we generally receive the infodump. If he monologues for 3 pages about how his father's brother's roommate's cousin from Nova Scotia ordering anchovies on their pizzas every Friday for 4 weeks in 1993 while knowing full well that he was allergic to fish led him to unleash that endless plague of self-replicating rabid marmots upon the city... No. Just no.
Put them all together and you have... The Dramatic Chipmunk Trifecta.
I will not call out any of the books that motivated this post, but any time you see a bunch of reviews amount to, "I loved almost the whole book, but then at the end..." it probably has at least two of the three going on. (Another peeve of mine is when there are 37 shocking twists in the last five pages, but that's another post entirely.)
Of course, there are exceptions to every rule. I just read a great book, Grave Dance by Kalayna Price, that technically hit all three of these issues. The implausible culprit was committing what turned out to be a fairly uncompelling crime for ridiculously complex motives - and yet the reveal was deeply satisfying because it illustrated quite clearly how the seemingly harmless acts of one of the main characters could easily get drastically, even world-endingly, out of hand. "Holy shit!" I said to myself. "No wonder his buddies have been so pissed off at him. What is Alex going to do about this?" The resolution to the mystery was less about what did happen than about the implications of what could happen if things continue as they have been. If you can incorporate all three of these things and I still come away with "Holy shit!' then I (and the chipmunk) salute you.
What sort of reveals irk you? Or are you fine with a little dramatic chipmunkery?