With this week's Cataclysm Beta build, Blizzard has added more optional assets to the client in the shape of upscale resolution textures for the game. These are not currently usable by players in the beta but are dataminable.
While normal WoW textures are 64x64 pixels, these new uprezzed textures are 256x256 pixels, or 16 times more detailed. Below are a couple of examples.
These textures represent an extra 30 gigabytes of storage space compared to the normal client. However, we datamined some strings in the first build that indicate that these will be optional, so most players likely won't need to download them if they aren't looking to use them.
Remember that Blizzard still hasn't announced anything definitive about upscaled graphics, but there were talks at BlizzCon about them, so we may see them in Cataclysm.
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Looking into them, it seems that these upscaled textures were generated using AI techniques, as some minute details, like small faces, were distorted in the process. Remember that this is only their first appearance in the client, so they are probably not final.
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These textures aren't limited to NPCs, but also apply to zones and cities. Below are examples of Stormwind, Orgrimmar, and Dalaran using these textures. Click on the images to enlarge them!