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.
Compare Aborted Arc 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.
No examples, please. This easily overlaps with What Could Have Been, as well as it also being a matter of personal tastes for some, and examples could easily get excessive.