Understand your Client
I think that to be a good t shirt designer, you need to be able to understand the client.
What is important is for the client to be pleased and to deliver exactly what the client likes. If you can nail this then you are a great designer. Because that is what its about. If the client doesn't get what they are looking for then they will go somewhere else to find it (another designer). SO get in your clients head first and find out what they like. Then when you know that, you can visualize it for them on a t shirt.
I am sure we have heard the saying "I get you", so get your client!