Anticipointment: High expectations followed by a letdown. A portmanteau of "anticipation" and "disappointment."
According to legend, the word was coined in the 1960s by a Los Angeles ad agency that used it as the punchline of a holiday card whose outside read, in paraphrase: "At this time of year, just one word sums up the feeling in all of our hearts." Now used for any instance of media or technology hype.
(I read the ad-agency story years ago and have been unable to corroborate it. Anyone?)