Obviously things can be broken down deeper, but in my mind, there were three key reasons that not only did we lose, but we got swept. I hate to harp on negatives at the end of the year, but I'll just do it quickly.
1) Pirahnas have no bite
Cheesy headline aside, the so-called "Pirahnas" did little to nothing this series. Luis Castillo, Nick Punto, Jason Bartlett, and Jason Tyner combined to bat .200, with a .289 OBP, and a .225 SLG. Pathetic effort over the 3-game series.
2) Bad hitting with RISP
I don't even know how things finished up here, and it's too depressing to look. All that you need to know is the Twins were 0/18 at one point, and their first hit with RISP was in the 6th inning of Game 3.
3) Bullpen Mismanagement
Joe Nathan, one of the best relievers in the MLB, pitched 2/3 of an inning this series. In Game 1, he should have come in in the 9th instead of Jesse Crain. In Game 3, he should have come in in the 7th inning when there were 2 on and 2 out. Although I suppose I can't blame Gardenhire... he's just doing what all managers do. And It's logic I can't understand. Biggest situation of the game, the Twins had to hold Oakland, and they go with their 4th or 5th best RP. But hey, at least Joe Nathan's rested!
Anyway, not a good series all-around. Oakland played very well, but the Twins practically handed them this on a silver platter. A disappointing end to a great season.