Last year the Yankees had the second worst defense in the league. The main culprits were Jeter, Cano, Giambi, and Abreu. So half the team basically. So far this year, the Yankees actually have one of the better defenses in the league. What exactly has changed though?
Well first off, replacing Giambi with a gold glove caliber first baseman has helped tremendously. While obviously it's helped the first base position it also appears to have had some kind of ripple affect on Cano. Cano no longer has to worry about a complete lack of range from the first baseman. This has allowed Cano to play further away from first base, like he did pre Giambi. Also, Tex has made Cano look even better by getting some balls that Cano clearly would have missed. Second, Derek Jeter seems to have rediscovered some of his range. He is definitely making plays this year which he would have had no shot at making last year. Who knows maybe he'll actually be able to stay at Shortstop until he retire. Lastly, the center field defense of Gardner/Melky has been way above average so far this year.
That's not to say the Yankees defense is fool proof though. The worst defender on the Yankees so far has been Nick Swisher. The real problem is that Nick Swisher is not a natural outfielder; he's actually a natural first baseman. Due to his unfamiliarity with the outfield he's often seen taking bad routes towards fly balls, which will often fall before he gets to them. The other issue defensively is A-rod. It looks as if his age combined with his hip surgery has slowed him down significantly. He's just not getting to line drives he used to, and with his hip really can't field bunts anymore. Oh, and pop ups still seem to be a big issue.
It's nice to see a good defensive Yankee team for once. The best part about it all is that this core is here to stay for the next 3+ years.