Regarding all the talk about gear re-use, I've taken the time to compile the data across all four expansions.

This is looking at gear from a WAR point of view.
Explanation below the image.



Link if images doesn't load: https://imgur.com/gallery/8JVr3JK

Purple: First time the gear model was used, never used since
Green: First time the gear model was used, later re-used
Blue: Re-used gear, but role-swapped
Yellow: Gear re-used on same role
For the re-used and swapped gear, I've listed the name of the original item.

Not including AF, tome or raiding gear as they even out across the expansions. While HW AF gear was tome2, the expansion did have the crafted Chivalric Coat set that was unique at the time (re-used in 3.05).

This does not look at dye, only gear models.

Based on this, we can see that the amount of new gear models is lowering from expansion to expansion.