Any team can win or lose any particular series, especially the League Division Series. It's kind of odd, actually, that the LDS is in theory the "least important" postseason series. Certainly, it doesn't carry the cachet of the League Championship Series or World Seriein While some folks may overlook it as a best-of-five series as opposed to a best-of seven, that is precisely the reason it can be so dangerous. The better team has a better chance of winning in a longer series, where they can prove their worth. For example, the Nationals, losers of 103 games, managed to beat the 102-game winning Yankees two out of three times in 2009. If the worst team can win a three-game series, it wouldn't be a stretch to think that any one playoff team could beat another in a five-game series.
A quick examination of the LDS since its inception in 1995 reveals that there is very, very little room for error. Of the 56 series from 1995-2008, the game one winner has won the LDS 40 times. So basically, for teams that slip up and lose their first game, 71.4% of the time they will lose the whole series. From 2007-2008, every team that won their first LDS game also won the series. It may not be so much that a team won its first game as opposed to the fact that they won a game. One win has a much bigger impact in a best-of-five series than in a best-of-seven series. The odd thing is, this seems to result in each individual LDS game having for of an impact than any given World Series game.
As the LDS is the crucial first step in the playoffs, it seems to me that it would make more sense for it to also be a best-of-seven event.