jack of course promised his future/ex-wife that he could fix her, but he couldn't, not really, not without whatever miracle happened (that may or may not involve desmond, but desmond brought up the "what if you could fix her").
michael couldn't keep the implied promise of being a parent for walt until recently (and i'm sure it wasn't just implied but i can't remember if or how michael said he'd take care of walt but couldn't).
sawyer promised to find the man who conned his parents and instead became that (type of) man.
charlie's older brother broke a promise to protect him.
maybe promise isn't the right word, it conjures up too much of a concrete, explicit agreement "i promise to..." but there are a lot of unspoken promises. locke was conned by his dad, who abandoned him when he was born anyway, more broken parental "promises". jin and sun took wedding vows (promises) which they couldn't keep to each other, jin because he got sucked into his father in law's world, and sun was going to leave him. kate, didn't she promise something or another to her childhood boyfriend? i don't remember.
anywho. it's not very well thought out but it's been brewing in my mind for weeks.