Comment Seems Solid (Score 5, Informative) 262
Currently, it only supports a subset of the features that JPEG has. It lacks support for any color representation other than 4:2:0 YCrCb. JPEG supports 4:4:4 as well as other color representations like CMYK. WebP also seems to lack support for EXIF data and ICC color profiles, both of which have be come quite important for photography. Further, it has yet to include any features missing from JPEG like alpha channel support.
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Every image format that becomes “part of the Web platform” exacts a cost for all time: all clients have to support that format forever, and there's also a cost for authors having to choose which format is best for them. This cost is no less for WebP than any other format because progressive decoding requires using a separate library instead of reusing the existing WebM decoder. This gives additional security risk but also eliminates much of the benefit of having bitstream compatibility with WebM. It makes me wonder, why not just change the bitstream so that it's more suitable for a still image codec?