Tiger introduced a really neat feature that gave every Cocoa app an instant, inline dictionary. Holding cmd-ctrl-D while mousing over a word causes a pop up to appear with the definition. If you keep holding down the key combination and move the mouse, any word you hover over is displayed. If the word wasn't in the dictionary, it would say so. However, often times this feature would just downright not work. I'd hold down the key combination while moving my mouse around and nothing would appear. At least it was easy to tell when it was broken. Leopard's dictionary lookup is faster and more reliable, but unfortunately takes a step backwards and no longer tells you when it doesn't recognize a world. Instead, it just doesn't display the pop up. In these situations–having been trained for the last few years on the shoddier Tiger implementation–I'm siting there with my mouse hovering over a word like a dumbshit waiting for it to load. Grr.
Note: Titling this entry as "Pet Peeve #1" doesn't necessarily mean this is my biggest peeve with Leopard, nor does it imply future entries in the Pet Peeve series.