Worn Brake Pads

OK riders don't do this. Disc brake pads last very well but like all things they do wear out. The pad on the bottom of this picture has no remaining pad material at all. In fact the metal of the backing plate is severely worn and that is putting it mildly. This is dangerous. Not it might cause a problem but more of a case of I am surprised you haven't crashed already. the owner complained that their brakes were making a noise and not surprisingly, that they could hardly stop. The bike needed new disc rotors as well.

Generally speaking new pads (just the braking material not the entire pad) have a thickness of approximately 2mm. Shimano recommends changing them when they reach .5mm.  I tend to err on the safe side and change them once they reach  .75mm. I would encourage you to do the same.