Sharepoint - the property or field has not been initialized. It has not been requested or the request has not been executed. It may need to be explicitly requested

This error means that you are trying to get some property that you didn't request explicitly. Maybe it's the Title field you're trying to use. Look at the example from here:

       function retrieveListItems() {

        var clientContext = new SP.ClientContext('/sites/MySiteCollection');
        var oList = clientContext.get_web().get_lists().getByTitle('My List');

        var camlQuery = new SP.CamlQuery();
        camlQuery.set_viewXml('<View><Query><Where><Leq>' + 
            '<FieldRef Name=\'ID\'/><Value Type=\'Number\'>100</Value>' + 
            '</Leq></Where></Query><RowLimit>50</RowLimit></View>');

        this.collListItem = oList.getItems(camlQuery);

        clientContext.load(collListItem, 'Include(Id, DisplayName)');

        clientContext.executeQueryAsync(Function.createDelegate(this, this.onQuerySucceeded), Function.createDelegate(this, this.onQueryFailed));        

    }

    function onQuerySucceeded(sender, args) {

        var listItemInfo = '';

        var listItemEnumerator = collListItem.getEnumerator();



    while (listItemEnumerator.moveNext()) {
        var oListItem = listItemEnumerator.get_current();
        listItemInfo += '\nID: ' + oListItem.get_id() + 
            '\nDisplay name: ' + oListItem.get_displayName();
    }

    alert(listItemInfo.toString());
}

function onQueryFailed(sender, args) {

    alert('Request failed. ' + args.get_message() + '\n' + args.get_stackTrace());
}

It's the way to tell client object model what fields do you want. The interesting part of this example is:

clientContext.load(collListItem, 'Include(Id, DisplayName)');