Hmm, pretty much anything if you have a pedantic MOT inspector.
http://www.ukmot.com/mot_check.asp
My recent failures have included:
Bulging brakelines (front and rear)
Loose headlamp (all three of my cars have failed on this in past 12 months)
Headlamp aim
Not having a seatbelt stalk for front passenger seatbelt. The fact that there isn't a passenger seat in my
Ibiza wasn't an excuse. If there is a seatbelt, apparently there has to be a seatbelt stalk!
Tyres
Rust in floorpan
Brake lines fowling front strut