It's about making sure that the 'facts' are straight.
I find this to be very true with cinema. When watching a movie, if there's real facts getting botched, I lose enjoyment. An example is the first time I saw "Mr. and Mrs. Smith." The movie is about a married couple who both work for separate spy agencies. While the action is great, a good amount of humour and romance mingled in, the one fact that pushed it out of the 'enjoyable' category for me was the shoes that Mrs. Smith was wearing. Anyone who had to do sneaking around, running, and practical work could not feasibly accomplish much in her shoes.
It's the little details that can ruin the overall image.
So when making fantasy worlds too; make sure the basics are in order. If something happens, it has to happen each time the conditions are met (when water condenses, clouds form). If one time, clouds do not form... why? It can't be "Because it didn't." I need a reason. And that's what makes believable fantasy world. You can have your dragons, centaurs, unicorns. I don't care. I just want to see a working, functional, believable world where the things that need to make sense, do.
"It's fantasy," doesn't excuse the basics.
So, bring the whole herd of three-legged imps and get to it!