I think the definition of the word "disappointment" is important here. Sean Taylor's death wasn't disappointing to me... It was a crushing moment for the franchise and a very sad moment for the fanbase. Personally, I don't think it belongs on the disappointment list.
To me in order to make this list there first needs to be a reasonable expectation of good things prior to becoming a disappointment. Also, for me disappointments are akin to regrets... Things you'd like one more shot at doing.
With that in mind, I echo others with Super Bowl 18... That loss kept the Redskins of the 80's from being considered a dynasty... And it kept the 1983 team out of all time greatest talk. The team that took the field in Tampa that night just wasn't the team we saw all year. And Gibbs was terribly outcoached by a Flores, which just shouldn't have happened. Over 30 years later, it still leaves a bad taste in your mouth. The definition of a disappointment in my book.
I'd toss in the 1997 season... My most frustrating and anger-fueled season I've ever had as a fan. I was so excited for that season and there were many teases that year that validated my excitement. But we blew games that year in ridiculous fashion.
Poor Deion....he gets blamed for our failures in 2000, but no one ever mentions the fact that we went from 28th in defense in 1999 to 4th in 2000. Deion, Bruce Smith, Carrier.... Those moves worked; and Jeff George was better than Johnson that year as well. Our failures were due to Johnson's lousy year and incompetence from Norv.!