Stock sizes against your walls
Home-store cabinets come in fixed widths. In a kitchen whose walls don't land on those sizes, the gaps get filled with strips of trim, and corners often waste space. A contractor can plan the run to your room and use every inch.
What they're made of
Many budget cabinets use particleboard boxes, which swell and crumble if a sink leaks. Plywood or solid wood boxes cost more and hold screws and stand up to water far better. Ask what the boxes are made of, not just the doors.
Who installs it, and who answers for it
Buying cabinets from a store and hiring an installer separately means two parties, and if something doesn't fit, each can blame the other. When one contractor plans, supplies and installs the kitchen, there's one person responsible from start to finish.
Don't forget refinishing
If your cabinet boxes are solid and the layout works, refinishing them with a sprayed enamel and new hardware can give you a new-looking kitchen for far less than replacing them.