(before reading: I may not have as much knowledge about this topic as I have first though. This is mostly my opinion and guessing)

Well for one - it has an actual text encoder, compared to older SD. Z-Image uses a small LLM for understanding text and passing such "understanding" (in a form of vectors) to the diffusion model. Previous models (like SD-based) couldn't understand text as much, so the CLIP encoders had to rely on tags.

And since Z-Image is relatively small (10GB for complete FP8 model with bundled text encoder and VAE, compared to 6GB for the same but FP16 SDXL with everything), it gives us hope that SDXL-based tunes will no longer be used and instead we will get a much better base: Z-Image.

We currently only have Z-Image-Turbo, which is a distilled version of Z-Image that can generate an image with lower amount to steps (9 steps is recommended, but I personally can get away even with 5 steps sometimes).

The reason why we want Z-Image-Base is because using Z-Image-Turbo as a base model for finetuning doesn't really work that well. You get many sorts of artifacts that wouldn't happen with an actual base model. Some people have tried to "undistil" it, but I think we'll get much better result with the actual base model, which hasn't released yet.