"The ranges of these variables must be discussed."
"The graphs must be properly normalized."
"The quantitative aspects of the work must be approached more rigorously."
If you want to tell us to do something, just say "The authors should...". It's perhaps a minor point, but I think it humanizes the discussion. If you actually write having in mind that there's a living, breathing person on the other end rather than just some science robot, then I think the tone will overall be much more constructive:
"Can the authors discuss the ranges of these variables?"
"The authors should normalize their graphs appropriately."
"The authors should improve the rigor of the quantitative aspects of their work."
Also, it tends to sound less absolute, which I think is nice because it gives the authors a bit of the benefit of the doubt. I've often misunderstood aspects of a paper as a reviewer, and it makes it easier for authors if they can engage a more ambiguous comment rather than have to somehow refute a very absolute sounding but ultimately nonsensical reviewer comment. You know you've gotten some of those! In fact, these days, it seems like those are the only ones we get...
I also remember once being on a thesis defense committee where one of the committee members kept badgering the student with annoying questions posed in this absolutist, passive voice way:
"The quantities in your graph must be discussed."
"The credit must be ascribed appropriately."
What a jerk!