There's nothing worse than finishing a book and feeling like you've wasted your time. For me, the biggest letdown is when a story builds up this incredible premise but completely fumbles the execution. Like, you get hooked by the synopsis—maybe it promises a dark, twisty mystery or a heart-wrenching romance—but halfway through, the plot collapses into clichés or the characters make decisions that make zero sense. I recently read this one fantasy novel where the magic system was brilliantly unique in the first act, but by the end, the rules kept changing just to suit the author's convenience. It felt cheap, like the writer didn't respect the reader's intelligence.
Another thing that ruins a book for me is when the pacing is all over the place. Some stories drag on for chapters with nothing happening, then rush through the climax in a few pages. Or worse, when the ending is clearly rushed because the publisher demanded a deadline. I remember picking up a highly anticipated sequel, only to find the last third was clearly cobbled together—loose threads left dangling, character arcs abandoned. It's like watching a cake deflate in the oven after you've followed the recipe perfectly. Those are the books I end up donating immediately, because I know I'll never revisit them.