> - doesn't support UTF-8 checkbox emoji

That's a good thing. Its also unicode, not utf-8. Your plugin can display it with a unicode character. Having a simple ASCII char set helps portability _a lot_. No benefit in supporting unicode code points.

> - doesn't support nested lists

If you can tick all items within a nested list, why bother writing it with dedicated TODO items? Additionally, you can just create more files for elaboration. Your file system is at your dispense. Additionally, that can be implemented in your plugin of choice. Goes with your first point in hand.

> - specifies a specific space character when a character class would be better

ASCII has no official character class. If your are refering to tabs; I don't think this is bad or good. Maybe you can follow uo on the use case..

> - not clear what "item must not contain blank lines" means

Probably that a blank line is used as a separator

> - description indentation limit of four space characters is a problem for nested items

I don't see why. The fixed amout of four characters obviously lines up to the start of the text from an item. This helps readability a lot. And is a simple standard for identation rules.

> - description supporting blank lines (properly indented) would be useful for longer descriptions

I disagree. A TODO list should be simple to grasp. Details and elaborations can be put in a dedicated directoy and refered to. Also, it is a _very bad_ idea to work with invisible lines. Users are dump and quick to judge.

> - date should support an ISO or other standard date format

This is the most portable standard, but extended for human editing..

> - timezone is necessary for events happening in specific timezones or across timezones

I tend to agree. But the person hosting the list should probably be the one defining the time zone. But I haven't read anythinf about time zones in the primer. Nothing stops one to simply append it to a time. No need for the standard to define such.

I think it is well put and the first one which checks all the boxes for me. Congratz to the author.

I don't see why such a comment is #1.