The fast and simple list:
Improvements in Android 1.6:
-Google search widget is capable of searching content on phone as well as web, including: contacts, texts messages, and SD card contents.
-Improvements to the camera, mostly speed improvements.
-Ability to use VPNs.
-New battery meter which will show what processes are using more of your battery.
-Improvements to Google Market.
-Software support for location via wireless networks (aGPS)
Improvements in Android 2.0:
-Ability to sync multiple user accounts at once.
-"quick contacts" feature added: click on a contact's picture to get a pop-up menu allowing you to quickly call, message, e-mail, facebook, ect ...
-Native MS Exchange support.
-Improvements to the camera software, including digital zoom, and manual white balance controls.
-Improvements to the virtual keyboard.
-Significant browser improvements, including the ability to tap the URL bar to type a web address.
-Double-tap zoom support, and (allegedly) multi-touch support.
-Bluetooth 2.1 software support.
-New framework APIs support some newer apps such as turn-by-turn nav for Google Maps, and Google Goggles (among many others).
And these are just Android improvements, Sprint/Samsung would likely include phone specific bug fixes in addition to the OS update.
Clark