It is the common sense approach advocated all over Stack Exchange:
downvote if the question/answer is weak or shows lack of research/ effort, or downvote to highlight a serious factual or logical defect.
It is standard practice to spell out what is expected from an upvote/downvote -- in fact we do already mention in the help topic related to voting that
Voting up a question or answer signals to the rest of the community that a post is interesting, well-researched, and useful, while voting down a post signals the opposite: that the post contains wrong information, is poorly researched, or fails to communicate information.
In addition, as noted by Community Manager @Shog9 in a comment here, the help page for the downvote privilege already covers the intended purpose of downvotes pretty thoroughly and goes out of its way to advocate restraint, but (he adds) whether anyone reads it is debatable.
In fact, new members may not often read that page, nor have enough exposure to SE culture yet to understand the implications of downvoting; and as for those who do know their DV's, even experienced members can too often be tempted to cast opinion-based downvotes. So there can be no harm and only good in specifying at the 'downvote' action point, what is and what is not expected from a downvote, by adding a popup reminder such as
downvotes are meant to highlight defects in the post such as lack of research/effort or a serious fault of logic. Please do not use downvotes to express your personal disagreement with (or disapproval of) the author or the content of the post.
I know this might only be possible to address at the network level, but such a reminder should ideally be incorporated as a popup message that appears when someone presses the downvote button. The system can then ask you to confirm the downvote, or else the user should be allowed 2 minutes to reverse their downvote at their own discretion, if they realise that they actually downvoted for the wrong reason.
However, the way that's presented should not make it more laborious for a user to cast a downvote: I am sure SE will not implement any feature that tends to discourage/ complicate/ inhibit downvoting in general, because downvoting is as integral as upvoting to how the system evaluates, ranks and displays posts on the basis of relevance, accuracy and quality.