I agree; JPEG has more artifacts which is most noticeable in low-frequency areas, but preserves more detail in high-frequency areas.
All of the non-JPEG codecs seem to have greatly reduce artifacts in the low-frequency areas, but for some of them (VP8 in particular) it was at a significant loss of detail.
Apart from JPEG's blockiness and VP8's complete loss of detail, it looks like the main difference between codecs is which parts of the image have detail preserved. Some codecs preserve subtle sky texture at the expense of the trees, others do the opposite.
All of the non-JPEG codecs seem to have greatly reduce artifacts in the low-frequency areas, but for some of them (VP8 in particular) it was at a significant loss of detail.