Full disclosure I started in BC and ended with Panda.
It appears to me that every area, with a few exceptions, are all upgraded visually with every DLC.
Now it's not B grade to AAA with every DLC but it is enough to change the min requirements with every DLC.
They have boosted the maximum quality with every DLC, that I had anyways, to the point that you really notice the difference between a 2 core CPU with a NV 1020 vs a 4+ core 1060. Especially in certain PVP zones such as AV where you get a lot of people on screen at at time with a Laggy WOW server you used to be able to drop to under 1 fps. Jumping to a real CPU/GPU would boost that to 10-12 FPS. I mention this because if you try and save on a cheap GPU, be aware that it may not work, or just suck, with future DLCs
As Night Owl said the cutscreens really improved over the years, to the point that they are mini-movie quality. Visually anyways, the acting and script is still hit and miss.
The starting area may not be as visually appealing interesting but the visual quality is comparable with other areas So it's not like the starting zone is 8 bit graphics and the end game zones are current AAA game quality.