iTunes 4.5 has logarithmic volume leveling. What does this mean to you?
This makes it easier to adjust the volume throughout the entire range of the slider because the volume slider behaves more like the volume knob on a stereo.
The volume slider in previous version of iTunes adjusted the volume in a linear way. But human hearing doesn't work that way. Human hearing is logarithmic, so small changes to the volume slider on one end make a big difference in the volume, while big changes to the volume slider on the other end make little change.
The maximum volume remains the same as with iTunes 4.2.