See this JS Fiddle, your solution works fine: http://jsfiddle.net/ahchurch/S6dBE/1/
Not for nothing but in your question (not sure if it's a typo or you just haven't noticed) - $scope.postItContent is not what you're binding your message to. Your binding is {{postItComment}}.
var myApp = angular.module('myApp',[]);
myApp.directive('postIt', function() {
return {
restrict: 'AE',
replace: 'true',
scope:{
message: "@"
},
template:"<div>Received: {{message}}</div>"
};
});
//myApp.directive('myDirective', function() {});
//myApp.factory('myService', function() {});
function MyCtrl($scope) {
$scope.message = 'Superhero';
}