I think this year will provide an excellent case study on the value of defense.
I think by all definitions, the Reds were the best defensive team in the NL Central last year. While Rolen may be overrated by the national press, he's certainly still average to above average, while Votto, Bruce, and Stubbs were all probably underrated. Further, they are likely to see improvement from SS where Paul Janish is slated to replace Orlando Cabrera.
The Cardinals finished 2nd in the division last year and probably needed an upgrade in a number of spots offensively and defensively. However they were a very un-Cardinal like horrid in the field last year, and this offseason they've gotten worse.
Trading Brendan Ryan for Ryan Theriot* makes them worse at SS, trading Ryan Ludwick/John Jay for Lance Berkman likely makes them worse in RF. The offensive improvement MAY overcome this, but it's hard to argue they're going to be worse off defensively.
* I realize they weren't traded directly for each other, but I'm using the trading term loosely throughout
One area where they may improve is trading Jason Larue for Gerald Lair. Yadier Molina played in 136 of the team's 150 games before going on the disabled list last year. He played all 20 innings of the marathon tilt with the Mets. If the team feels more comfortable with Laird who is no slouch behind the plate, it may improve the defense from the catcher overall.
The Brewers last year scored runs in bunches, but they couldn't pitch and they couldn't field. They believe(with good reason) that they've improved the starting rotation. However they've taken a bad fielding team, and removed the starting shortstop and the starting center fielder. It's at the least debateable whether these will improve them defensively, but I don't think Yuniesky Betancourt will strike fear in the hearts of opposing hitters.