Great point, Dom. I really like what you said here. Thinking on it now, seeing Mario without his cap has always been a pleasing image for me also. I wonder where I first saw it - SM64, perhaps? I think it's because it humanizes him further. Those are the little touches I always like in Mario games.
Yeah I think it was 64 where we saw him without a hat. In his appearances in other games Mario either wore his signature hat or a hat that belonged to the profession that Mario was taking part in. Like a chef's hat in Yoshi's Cookie.