I wrote an article once on what makes a gamer tag 'good.' It was very fun to write, but it was also pretty well received. It basically just put into words what most people would think obvious: tags, numbers, alternating caps, X and Z characters by themselves, all those sorts of things making a gamer tag less respectible. Yeah I know everybody has a reason why their use of numbers and the likes is the exception so I get eaten alive everyone by those folks, but it's still a pretty safe general rule.