IGN has wrangled up a nearly 19-minute video walkthrough of how Diablo III's closed beta test has evolved since it kicked off last September. The video is complemented by some text as well, so I have some quoting to do:
The actual playable content hasn't changed since the closed beta's launch in late 2011. You still smash through an early part of the game then fight the Skeleton King. But many of the upgrade and stat systems have shifted around.
Blizzard still appears to be toying with how active skills are swapped in and out. In Diablo III, you'll eventually unlock six slots to assign active skills and three slots for passive skills. These function as your character build and are easily modified, letting adjust your class' strengths on-the-fly. Initially in the beta it was possible to swap out skills as often as you liked, though Blizzard was concerned this ease of switching encouraged you to play with the skill window open at all times. A later version required you to visit an altar in town to switch skills, locking out your ability to change skills in the field. In the current version of the beta, you can again swap skills whenever you like, though swapping triggers a 30 second cooldown during which swapping is blocked.
Many of the changes are still in-development and could be tweaked further before launch, but other things like the removal of Scrolls of Identification and Scrolls of Companion sound permanent, based on what Blizzard's said in official posts. For an overview of all the changes so far, check out the embedded video.