Good spec: XT or XTR derailleurs , rims with eyelet spoke holes, XT/XTR or boutique crankset, Magura, Pauls, Hope, Kore, Raceface, Syncros, Titec, RockShox, Manitou, Pace, etc
Crap spec: DX, Acera, DuraAce, Suntour, Cane Creek, RST, anything Chinesium, anything older than the frame, mismatched...