Others point out missing tz.

It's also not that "user friendly": depending on their locale, users will usually expect for instance DD/MM/YYYY. Sorting by YYYY-MM-DD won't feel natural to them.

I am french. Everydays, we use DD/MM/YYYY or DD/MM/YY. Sometimes, I encounter YYYY-MM-DD, for example at the beginning of a document reference or in a file name. For me, it feels natural and I have no issue to make this switch mentally. The only problem I encounter is in english: MM/DD/YYYY. Hopefully less and less people are using this insane order.

> Sorting by YYYY-MM-DD won't feel natural to them.

It's dictionary sorting.