It all depends how you tell them. If you are both stopped at a light, and you lean over, smile and say you are not really an expert, but that seat looks terribly high to you, and do they find it comfortable? If they say yes, then fine, move on, and if it gets them thinking about whether you are right, that's fine too. I stopped a complete stranger a while back to suggest that her tires were really flat (we were both waiting at a stop light), and she agreed that she thought they were, but she didn't have a pump. I did.

Thanks man... I just spewed a perfectly good gulp of Rickards Red.

I need my seat higher than normal because of my chronically bad knees. Unless I can fully extend my legs while pedaling, my knees ache in pain after riding. I would be offended by someone presumptious enough to tell me that my seat is too high, after my many years of experimenting with various seat and handlebar heights to alleviate the pain in my knees. I say, unless you know the person you are talking to, better keep your opinion to yourself...because until you know the person and their reason for doing what they do, your opinion on the height of their seat is just that.