While waiting for Legion beta, I've been looking at the earliest maps in previous betas/alphas that are available and it's fun seeing what changed.
Draenor is particularly fun because it's where we are now, so everything is fresh. It has the original plan for Gorgrond, where Iron Horde had a much larger presence and the Botani stuff was likely going to be in "Faralohn". You can also see Khadgar's place doesn't exist, and instead is the time travel device that we were going to get to Draenor with.
Pandaria didn't change much at all. Interesting note, the map name is HawaiiMainLand.
Northrend has a few changes. Storm Peaks is mostly unpopulated with things at this point, and Dalaran is a simple town on the floor of a rather multi-colored zone.
Outland is a good one. Blade's Edge is really rough, Black Temple is far less awesome, Netherstorm has no greenery.. Lots of subtle changes all over.
Bonus map:
Kalidar - Battleground which has been in since 0.5