Outland certainly had some environmental variety, but a lot of its environments were pretty bland imo. HFP and SMV are sort of like different colored versions of each other (minus BT which stands out in a good way). Netherstorm, again, barren, but the Manaforges and Ecodomes gave it just enough personality to stand out. Pandaria may not have as much variation but the design is pretty great, especially Jade Forest (my favorite zone).