My policy: be nice to people. Don't be nice to ideas.

IMO, ideas need harsh critique, and it's not always easy or worth it to spend the extra time padding the conversation to soften your point - it can even be counterproductive.

So while I won't say that someone is stupid or dumb for suggesting the idea, I may say an idea was not well thought out if it really isn't. I expect my peers to do the same, because we're not going to see which ideas stick if we bicker over how we iterate on them.