Convolution filters attend to a region around each pixel; not to every other pixel (or patch in the case of DiT). In that way, they are not good at establishing long range dependencies. The U-Net in Stable Diffusion does add self-attention layers but these operate only in the lower resolution parts of the model. The DiT model does away with convolutions altogether, going instead with a linear sequence of blocks containing self-attention layers. The dimensionality is constant throughout this sequence of blocks (i.e. there is no downscaling), so each block gets a chance to attend to all of the patch tokens in the image.

One of the neat things they do with the diffusion transformer is to enable creating smaller or larger models simply by changing the patch size. Smaller patches require more Gflops, but the attention is finer grained, so you would expect better output.

Another neat thing is how they apply conditioning and the time step embedding. Instead of adding these in a special way, they simply inject them as tokens, no different from the image patch tokens. The transformer model builds its own notion of what these things mean.

This implies that you could inject tokens representing anything you want. With the U-Net architecture in stable diffusion, for instance, we have to hook onto the side of the model to control it in various sort of hacky ways. With DiT, you would just add your control tokens and fine tune the model. That’s extremely powerful and flexible and I look forward to a whole lot more innovation happening simply because training in new concepts will be so straightforward.