Common problems with doors include draftiness, sticking/jamming, gaps at the bottom/top/sides of the door, and warping due to moisture damage. These problems can usually be fixed fairly easily by making adjustments to the hinges or weatherstripping (for draftiness), sanding down sticking points (for sticking/jamming), or adding trim (for gaps). Warping is usually more difficult to fix and may require replacing the entire door if it’s severe enough.