I recently remarked that the phrase "that doesn't seem obvious to me" is good at getting people to reassess their stated beliefs without antagonising them into a defensive position, and as such it was on my list of "magic phrases". More recently I've been using "can you give a specific example?" for the same purpose.
What expressions or turns of phrase do you find particularly useful in encouraging others, or yourself, to think to a higher standard?
depersonalizing the argument is something I've had great success with. Steelmanning someone's argument directly is insulting, but steelmanning it by stating that it is similar to the position of high status person X, who is opposed by the viewpoint of high status person Y allows you to discuss otherwise inflammatory ideas dispassionately.
This is not quite what you want, but if you are a grad student giving a talk and a senior person prefaces her question to you with "I am confused about...", you are likely talking nonsense and they are too polite to tell you straight up.
5gwillen
I frequently use "Hmm, it's not entirely clear to me that [X]...", which seems very directly analogous to yours.
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.