Mura CMS : This is an example of how to make the Summary field a required field. You could use this example to make pretty much any other field, including extended attributes, required as well.

muraRequiredSummary.cfm

<cfscript>
  // Drop this in your Site or Theme eventHandler.cfc
	public any function onBeforeContentSave($) {
		// reference to the newBean
		var newBean = arguments.$.event('newBean');
		// reference to the original contentBean
		var oldBean = arguments.$.event('contentBean');

		// example on how to create some errors
		var error = '';
		var errors = {};
		
		// Make Summary a required field
		if ( !Len(newBean.getSummary()) ) {
		  StructAppend(errors, {'e1': 'Summary is required.'});
		}

		// if there's errors
		if ( !StructIsEmpty(errors) ) {
			for ( error in errors ) {
				arguments.$.event('contentBean').getErrors()[error] = errors[error];
			}
		}
	}
</cfscript>

@Steve - the above code seems to throw an error: http://pastebin.com/TW9Mmmkz

This appears to be Railo only, Coldfusion baulks for invalid syntax.

This will get you going on Coldfusion:

<cfscript>

        public any function onBeforePageNewsSave($) {
            var newBean = arguments.$.event('newBean');
            var oldBean = arguments.$.event('contentBean');
            var error = '';
            var errors = structNew();
            if ( !Len(newBean.getSummary()) ) {
              errors.e1 = 'Summary is required.';
            }
            if ( !StructIsEmpty(errors) ) {
                for ( error in errors ) {
                    arguments.$.event('contentBean').getErrors()[error] = errors[error];
                }
            }
        }
    </cfscript>