Rather than jump into a condemning remark like "That is a stupid idea" I simply say, "I don't get it". Then I talk the idea through with them while trying to understand why they think it is a good idea.
Sometimes I find there was some data I was missing. But, if I still believe it to be a bad idea, rather than tell them their idea is stupid I simply say "I'd never build that because of x, y, & z" which is an honest and unassuming statement.
That's not what the OP is really talking about though, as what you've described is fair and constructive :)
People gossip. I'm not in the Valley myself, but friends are, and from what I've been told the OPs description of behind-the-back unconstructive negativity... Well, I think that's a bad way of helping each other.
To put it simply: I think we need to be a little more sensitive. That doesn't mean we can't be critical, but if you temper that with some empathy, you'll often get better results, in my experience.
Sometimes I find there was some data I was missing. But, if I still believe it to be a bad idea, rather than tell them their idea is stupid I simply say "I'd never build that because of x, y, & z" which is an honest and unassuming statement.