The BCS is the ultimate letdown in sports for three reasons.
1) In-Season Letdowns: Considering the nature of the BCS selection process, the "every game is a playoff" mentality is a major marketing tool for the BCS. However, it's a huge letdown and disappointment for fans when they lose a huge regular season game or get upset on a fluke and you know your team has no shot an the national title game anymore.
2) Selection Letdowns: Every BCS selection show triggers an onslaught of debate about who should have been playing who and who got left out.
Not a year goes by where a third elite team is left on the outside looking in on the national title game, or a small conference team gets a poor matchup or is deemed undeserving of a major game despite a gleaming record.
3) Postgame Letdowns: When the games are all played out and the scores are settled, there are major letdowns, mostly for the teams that won, ironically.
Just recently, Boise St. beat up on another mid major in TCU and their fans were happy, but disappointed that they didn't get the real shot they deserved. Likewise with Utah, who was given a chance to upset a major powerhouse and did so in style, which brought about questions as to if they should have been playing for more.
The BCS is all about perception, with what teams the computers perceive to be the best teams on paper. We all know perception isn't reality, especially in sports and as a result, the BCS continues to be a huge letdown every single year.