Honestly my biggest complaint is header wrapping. Technically you need to wrap lines at 998 bytes (not that any reasonable server actually cares). But in order to wrap a header you need to add spaces (because you can only break a line after whitespace). But where spaces are unimportant depends on each specific header. So you need to have custom wrapping rules for each header.

In practice no one does this. They just hope that headers naturally have spaces or break them in random locations (corrupting them) because the protocol was too stupid.

Binary protocols are just so much simpler. Give the length, then the data. Problem solved. Maybe we could even use a standard format for structured headers. But that would be harder to do while maintaining backwards compatibility.