For me a few I'm afraid:
CO - the complaints about lack of international upgrades made me give up
NW - Oh that's right I was forced in to Flying Dutchman
TW - Oh yes they went broke
BA - The UK scheme is soooo mean that I credit flights to AA or QF now
AF - Who wants to fly AF anyway
DL - never fly them so used up 50% of miles on a F trip from LHR to JHB.