Changes

It's possible that one of them opens the [[active skills window]], and the lower left icon is a possible match. The lower right one shows a scroll, but we've already got a quest icon, so perhaps it's the return of the message log? That button existed in Diablo 2, but was not previously shown in Diablo 3. This button looks nothing like the D2 version, though.

Changes from this one to the most recent version aren't major, but they are visible. There were initially 1-6 hotkeys, but 5 and 6 could only be used for potions or scrolls. The RMB, LMB, and Tab worked the same, but were not yet labeled on the interface. The game option controls for trading, multiplayer options, and more were previously on top, as they were in Diablo II. In more recent versions they've been moved to a small rectangle on the far right.