People hate not knowing what something says and are naturally curious creatures – they will crane their necks to see what it is they are missing.
Finally though, and arguably most importantly, it needs to be something that people are actually going to wear, and here you have to know your audience. You are not going to create custom clothing that people are willing to wear to smart events and however well designed it is, it will be something they keep for rough occasions – after all it was probably free or at least very cheap. As such your t-shirt should appeal to this kind of behaviour and that’s one reason that personalized golf merchandise works so well. For example give away promotional polo shirts and they will get worn during sports – when most people aren’t concerned about how they look.