AHA! Whenever someone talks about OSTs, I am there! ...At least on this site. ...When I notice it. ...And can be asked.
Well, there is an interesting discussion to be had about what makes an OST the worst. Eargrating and annoying or forgettable and without impact?
I think that an OST doesn't fully works on his own. It may be a beautiful piece of music, but the full effect is only achieved in conjunction with the game's content it accompanies, and it is that effect that sticks with us everytime we hear it.
For example, by playing an unsettling, strange-sounding OST during a bossfight, this will affect the player and make him more tense. The OST is not something you would listen to on your own, but it does what it sets out to do by creating a certain mood the game designers where going for, so I wouldn't call it a bad OST. A bad song maybe, but not a bad OST.
So logically, I think a bad OST is one that fails at what it is made for, strengthening a scene by affecting the player's mood and feelings.The songs that we barely register and phase out completely after a while. I would post an example, but - and this is not a joke, honestly - I can't remember any.