From Merriam-Webster's:
Faith: a (1) : belief and trust in and loyalty to God (2) : belief in the traditional doctrines of a religion b (1) : firm belief in something for which there is no proof (2) : complete trust
The first one is what many on JWD express. They have faith in God as a being. Not faith that he'll do anything in particular, or that he demands anything in particular, but simply faith that he's there. In some cases, that he cares. Other than perhaps leaving a person with some unanswerable questions, this doesn't seem unhealthy to me.
The second is more damaging, because it specifically requires a person to ignore facts that contradict the "traditional doctrines" of their religion. That's pretty much where we were as JW's.
The last two get into solid gullibility, in my opinion. You are just setting yourself up for a fall if you have "complete trust" in anyone, let alone having "firm belief" in something unproven.
But then "proof" means different things to different people.
Dave