I've been trying to use something closer to ColinDabritz's second wording: "What am I missing that makes ____ unreasonable?" or "I don't understand why ____ wouldn't work". Phrasing it like this hopefully conveys both that I'm aware that there's a gap in my understanding of the problem, not our solution to the problem, but still elicits a response more directed at my particular understanding. It's also easier for me to formulate what I don't understand as "here's what seems obvious, why is the solution different?" than "I don't understand this particular aspect", since I don't know the part of the solution to ask about.