As the Yankee dynasty melted down after 2000, one of the most glaring annual shortcomings was the lack of attention paid to defense. The Yankees plugged big bats like Bobby Abreu, Gary Sheffield and Jason Giambi into the lineup, and watched singles turn into doubles thanks to the inability of those players to field their positions well enough. That's why the decision to go with Brett Gardnerin center field is a significant move toward repositioning the Yankees.
Gardner's speed should help him run down a lot of balls that fell in for hits across the last decade, something that will help their expensive rotation live up to their stature. That's worth whatever the Yankees lose in terms of run production, although Gardner will still need to do something offensively to keep the job.
People have wrongly compared Gardner to Dustin Perdroia, mostly because it's easy to slap every smallish white guy with the same label. Gardner's never going to win a MVP award, but he can help the team offensively nonetheless. If he keeps his on-base percentage in the .340-.350 range and doesn't make outs on the basepaths the Yankees will be able to put a lot of pressure on pitchers with the top of the order.
If he can't do that, though, the Yankees will feel pressure to make a change no matter how well he plays defensively. Melky Cabrera isn't going anywhere unless/until someone makes an offer Brian Cashman can't resist, but Gardner was never really competing against Cabrera for the job. He was competing with the potential of approved veterans like Mike Cameron and the expectations that will be hung on anyone playing center for a team with as much on the line as the 2009 Yankees.
Giving him the first crack is a step forward for the Yankees, but that's just half the battle for Gardner.