In the current situation, a ContentValidationException exception is thrown (https://github.com/ezsystems/ezp-next/blob/master/eZ/Publish/Core/Repository/ContentService.php#L542) when the field is marked as required and the field has not been set at all.
However, setting it with empty content will not trigger it:

$content->setField( "requiredTextLineField", "" );

The method isEmptyValue() (https://github.com/ezsystems/ezp-next/blob/master/eZ/Publish/Core/FieldType/FieldType.php#L258) is used to test emptyness in a very generic way and is until now, never overridden:

public function isEmptyValue( $value )
{
    return ( $value == $this->getEmptyValue() );
}

Some field types' getEmptyValue() returns null, some returns a Value object.

The values used for emptyness has been summarized in a document: https://docs.google.com/a/schlitt.info/spreadsheet/ccc?key=0ApxQ-KCWjCMBdFItWlRuQ2ptZFNsNFFOb3VHZ1BXR2c but it looks out of sync with the ones returning a Value object.

Approach 1: Shouldn't all getEmptyValue() return a Value object? Which can be used by isEmptyValue() to compare with the == operator for most Field Types? And more complex ones would override the isEmptyValue() (XmlText for example).

Approach 2: Should the current return values of getEmptyValue() be preserved and isEmptyValue() surround calls to getEmptyValue() with acceptValue():

public function isEmptyValue( $value )
{
    return ( $value == $this->acceptValue( $this->getEmptyValue() ) );
}

Approach 3: Nor getEmptyValue() nor isEmptyValue() should be changed, isEmptyValue() should be implemented for most field types.