When hearing ideas for the first time, it's important to listen without being judgmental. If you from disagree too strongly early on, many people will find it easier to just let you "win" the argument. Not only does this squash potentially good ideas, it'll also lead people to think twice about whether it's worth sharing ideas with you in the future.
"Yes and..." exercises are helpful, but they're also time consuming. To figure out whether it's worth investing time in an idea, you need to fully understand it before starting to look for flaws. Remember that understanding the motivation for an idea is often more important than the idea itself.
This has similarities to my thoughts on honest feedback.