oh boy...I agree on the chrome parts attached in bad spots. also, clear parts can suffer from bad sprue design. engine halves that don't line up no matter what you do, and tires that have severe marks on the tread from the mold, or got misaligned completely.
also, radiator hoses are a pain in the butt no matter what you do but my biggest pet peeve is when I screw up and don't notice until it's too late. glue on a body panel, scratches on the windshield, cutting a part too deep, sanding too much from a body panel, or having the whole darn thing put together before I realize I missed a bunch of mold lines.