Samsung has done some cool stuff to Android for the Captivate on AT&T. First and foremost, you can have up to seven homescreens, or you can have just one. Meaning, you can add and delete homescreens depending on how many you want to use. HTC should take note of this feature! Common to every homescreen view are the four docked icons on the button which sadly can't be changed. Update: yes, you can change the docked icons! If you go into the application tray and hit the menu button, there is an edit option. Nice!!
Speaking of the notification bar, Samsung has added shortcuts to control WiFi, Bluetooth, and the phone profile, right from the notification bar. Also, when you're on a call, you'll get a new element in the notification bar that lets you end the call from any screen.
Samsung has added a handful of useful widgets beyond what you get from stock Android. There are widgets for social networking, stocks, weather, calendar, and more, although you can always download additional widgets from the Android Market.