Not sure if it's a hardware issue, but for me the biggest annoyance with my 7D is the DOF-preview button. Now I know they've moved it in the 1DX and 5D3 presumably too.
But even if it's a button, no matter where it is, and no matter how stable your tripod, it's still useless for really really small macro work, where you definitely need a dof-preview button, same with landscapes but there the smallest vibrations don't muck with your framing as much.
It doesn't have to be a dedicated switch, just a menu-option like MLU is currently is good enough for me. But it's stopping me from ever buying an MP-E 65, and until there is such a feature, i'm sticking with old/manual lenses for tabletop maro work...