L Doors that are too small. L Deeply embedded pee of previous occupants. L Dried urine and poop on the floor mesh. Use a wire brush to scrub off rust. This includes the cage bars and cage floor.
Plastic is much easier to scrub clean than metal.
L Wire floors bad for GPs. L Wood sides and top don't provide adequate ventilation or sunlight. L Top lids that flip open, and thus can be opened by raccoons and possums, if the cage is outdoors. The legs can scar your floors, if the cage is indoors. If the size of the cage is inadequate for your pet, add a shelf so that he or she can hop up on it, getting a little more exercise and room to stretch.