So the next time somebody shows you something that looks really great, do this...Do Nothing!
Really, just appreciate it in a mild mannered way (or applaud it a lot, depending on your style), but do not discuss about whatever else it can be or how it can be so much better.
From a first hand experience of being on the other side of the fence (i.e. the guy who thinks he may have the next cool thing), nothing kills the potential of a fresh (& nascent) new idea than some discussion on what else it can be, especially when you may not have a direct sweat equity in it.
Ideas are like little babies, yes...they may grow up to be something great (& you can talk all day about their career options), but their mother would rather have you just appreciate how cute & cuddly they are, and maybe play with them a little.
Also, at the cost of sounding all gooey, such discussions inadvertently take away the euphoria of creating something cool & refreshing.
Having said all the above, constructive criticism, vigorous intellectual debates & a large dose of left brained analysis is very very important, but it has to wait for the right time in the conceptualization process. Often, these input can prematurely burden the evolution of a concept with more food for thought, & potentially delay a finished version of the prototype itself.
Like Steve Jobs said, "Real Artists Ship". Be the real artist or find one & let 'em ship.
Prototype, Love, Iterate, Peace ;-)