Exhaust System
Complete, secure, correct type and not too noisy.
Its pretty vague which is why its mostly down to the testers discretion. I have found a friendly MOT tester near me who is OK with noisy exhausts, as long as you don't take the pi55.
I mentioned Harleys in another post. My mate has a Fat Boy with straight through pipes. They go straight through the MOT as the tester believes they are not too loud. After working with custom Harleys for years he is probably stone deaf.