The big change is the addition of the Slam Zone. Before each pitch, you guess location. Unless you turn the option off, you must always guess a location, because not moving the Thumbstick "guesses" down the middle. But don't worry, there is no risk at all for guessing wrong. Should you guess correctly, spot on to the location, or if the pitcher totally hangs a ball, you enter Slam Zone.
This puts you into a cinematic view focusing in on the ball leaving the pitcher's hand. As it travels in slow motion, you tap the appropriate button furiously to fill your power meter and hit the power swing button when the "swing" icon flashes on screen. The result is almost always a home run. Granted, it's a cinematic feast, but it does feel a bit like a gimmick in its current state.
As it stands, a few months from release, Slam Zone is a little too easy to achieve and seems far too easy to rock homers with. However, there's plenty of time for tweaking and since this was the only major complaint from the three editors playing and loving MLB 2K5, I imagine Kush and VC are taking the issue seriously.
The real problem, as far as my short experience has shown, is that pitch guessing has no risk. If you guess low and inside and the pitch is high and away, there's no penalty for the batter. Add to this the fact that pitcher's can bow to pressure, but hitters only have to worry about confidence (which also affects pitcher's). Home run hitting is always a touchy balance and my main concern is that there's too much in the hitter's favor right now. And since the Sox managed to score 17 on the Yanks through many a long ball, there's still some tweaking needed to be done. Fortunately there's time. And, of course, if it does end up being too easy to abuse, Slam Zone can be turned off in the options.