Often when people ask, "What do you think?" they're looking for validation instead of soliciting a genuine opinion. For small things, like giving feedback on someone's outfit, it's not a big deal if we disagree. But, for larger decisions, like deciding whether to move in with a significant other, this becomes a much trickier issue.
Instead choosing between sharing my true feelings and offering even tacit validation for something I disagree with, I'll ask, "Do you really want to know what I think?"
By asking the question, I've hinted that I don't agree without actually saying so; with that in mind, people can choose whether they want to hear me out. If they do, they've had a chance to mentally prepare themselves and are less likely to get upset. And if they don't, I've provided them with an easy out.