Shirley and Stan are definitely right points. I will define a good tee is: 1)nice cut design(fit to your market people); 2)correct fabric material(not best, but correct, personal favorite is different); 3)logo design added to create the uniqueness of your tees 4)any additional decorations that most tees don't do currently to give you another selling point. The pre-condition is the overall quality has to be perfect, good at least.
Just my 2 cents word. Thanks.