There's an article about this at Baseball Prospectus that I recommend you read. The idea is that the Rays' massive improvement this year has to do with an incredible improvement in their run prevention, and that improvement can largely be credited to the changes they made on their defense.
Last year, the Rays won 66 games; this year, they've won 92 with eight games to play. Last year, they allowed 944 runs, this year, it's 632. As the BP article points out, the improvement they're going to make over last year in that category is historic. The acquisition of Matt Garza has something to do with that improvement, but the 2008 Rays are using a lot of the same pitchers they had last year. The big difference is that the 2007 Rays had the worst Defensive Efficiency of any team in BP's database, while the 2008 Rays had the best Defensive Efficiency in baseball.
That's incredible, and what's equally incredible is how little tinkering it took to get it done. Five of the eight principal Rays fielders this year were also among the eight main fielders in 2007.
The Rays dealt shortstop Brendan Harris and right fielder Delmon Young to the Twins for Garza and a far better defensive shortstop, Jason Bartlett. They moved Aki Iwamura from third to second, where he replaced B.J. Upton, Harris and Ty Wigginton, and replaced Iwamura at third with rookie Evan Longoria. Upton moved to center field full time, and Gabe Gross replaced Young in right. That's all--one major trade, one minor trade (with the Brewers for Gross), one promotion, and some position shuffling.
The Brewers have been similar, if less extreme, in that they've made serious gains in run prevention by making some relatively easy moves to improve their defense. They finished 25th in baseball in Defensive Efficiency in 2007 and allowed 776 runs; this year they're sixth and they've allowed 672 with just six games to play. All they did was sign Mike Cameron to a relatively cheap deal, move iron-gloved Ryan Braun from third to the outfield and Bill Hall from the outfield to third.
What both these examples show me is that once you have talent, sculpting it into a real ballclub is relatively easy. Braun had terrible hands at third, but he was a good athlete and a great hitter, so, poof, he became an outfielder. Upton was a great athlete and a good hitter who turned out to be a bad infielder, so the Rays addressed the problem by moving him to the outfield and using their excess of outfield talent to acquire a good defensive shortstop and a top-of-the-rotation starter. Suddenly, they'd fixed their problems--their players were finally in positions that suited them, and their glut of outfielders and attitude problems was gone.
And so when someone says, for the millionth time, that if the Bucs trade Jack Wilson they must get a shortstop or they mustn't get another third baseman or outfielder, don't listen to them. Yes, it's incredibly painful to have to watch Luis Rivas play baseball, but what the Pirates need isn't a shortstop, it's talent. I'd much rather see the Bucs acquire a couple of young arms for Wilson than a marginal shortstop who happens to be closer to the majors. Once the Bucs accumulate a ton of talent the way the Rays did, then they can acquire their Jason Bartlett.