PDF version 1.7 by default?

[Karl Berry]

Ulrike tells me the PDF Association is now recommended that the default
PDF output be version 1.7, and suggests that TeX Live follow suit. Right
now our formats are set up to output PDF 1.5 by default.

Does anyone know of specific practical compatibility problems that might
ensue if we switch to 1.7 by default? I know there can be problems "in
theory". But does anyone know of resulting problems in practice?

While I'm here, FYI, Ulrike also says that for LaTeX

> We will default to 2.0 if \DocumentMetadata is used, as this is better 
> when the PDF is tagged

Thanks,
Karl