You feel like you've been given the first three chapters of a story, a story that you really would have liked to see to its conclusion, but either a) it's on a whole different set of rails now, or b) while you do see it to its conclusion, it ignores the intriguing possibilities you'd hoped to see. Note that this does not include those plots/ideas you dislike in an otherwise well-written story/well-made game/etc - nor is it a "Valid criticism" as this easily steps into rewriting the work.
Compare Aborted Arc, What Happened to the Mouse?, Fan-Preferred Cut Content and Worse Than It Sounds. Not to be confused with Offscreen Moment of Awesome, where the thing you would have liked to see did happen but only implicitly (they never actually showed it). Often results in Alternate Continuity Fanfic Fuel. For this applying solely to characters in the plot, see They Wasted a Perfectly Good Character. See Underused Game Mechanic for examples pertaining to functional video game elements.
Has nothing to do with Intended Audience Reaction, otherwise known as They Plotted A Perfectly Good Waste.
No examples here, please. This easily overlaps with Fan-Preferred Cut Content, not to mention it is a matter of personal tastes for some, and examples could easily get excessive.