Contacts now let you search an entire record for a term, not just the name fields. This is terrifically handy if you want to add a keyword to help refine searches when you're looking for contacts whose names may not be on the tip of your tongue. The calendar apps tweaks are subtle, to the point that you might miss them if you blink. But the graphical fixes (for example, adding a "Today" label to the button you tap to return to today's date on the calendar) enhance usability.
Improved: Navigation
The Home screen button now returns you to the last home screen you accessed. The quick access button--one of the three main navigation keys at the lower left of the screen--now lets you scroll through your recently opened apps. The most recently opened app appears at the bottom of the list, with the older apps scrolling up and off-screen. Still, the finger-flick access makes sense--though I still find I ultimately may need to move my fingers more to use the so-called "quick access" feature than if I had just reopened the app from one of my home screens.
Improved: Android Market
Again, some subtle interface tweaks really help boost usability. For example, now the version date of the app appears right next to the version number. The addition of movie rentals at the top expands the Market's functionality.