Plumbing. I'm not a plumber and I'm not particularly good at it, but it's one of those things that most people won't even consider looking at.

Also, 3D visualization. I had a carpenter do the gutters on my house and I explicitly told him that the reason I didn't do them myself was that the eaves are slanted inwards so that the slope on the gutters would cause the gutters to go inwards when it goes downwards, and I was unsure what best practise was for that case, where to get the proper hangers for this or if we'd need to put a vertical board up first in order to make it work. He assured me that it'd be fine, having done many gutters before. When I got home, he had put ordinary hangers right on the slanted eaves, and the gutters were halfway under the roof at one end. He stood there scratching his head and tried to argue that the wall of the house was not straight, because he could simply not see any other reason for it to do that.